Zodiac Killer is the pseudonym of an unidentified serial killer who operated in Northern California from at least the late 1960s to the early 1970 s. The Zodiac murdered victims in Benicia, Vallejo, Lake Berryessa, and San Francisco between December 1968 and October 1969. Four men and three women between the ages of 16 and 29 were targeted, with two of the men surviving attempted murder. The Zodiac himself claimed up to 37 victims. The killer originated the name "Zodiac" in a series of taunting letters sent to the local Bay Area press. These letters included four cryptograms (or ciphers). Of the four cryptograms sent, only one has been definitively solved.^[2]

Suspects have been named by law enforcement and amateur investigators, but no conclusive evidence has surfaced.

The San Francisco Police Department marked the case "inactive" in April 2004, but re-opened it at some point prior to March 2007.^[3][4] The case also remains open in the city of Vallejo, as well as in Napa County and Solano County.^[5] The California Department of Justice has maintained an open case file on the Zodiac murders since 1969.^[6]

Although the Zodiac claimed 37 murders in letters to the newspapers, investigators agree on only seven confirmed victims, two of whom survived.

They are:

The following murder victims are suspected to be victims of Zodiac, though none have been confirmed:

There is also a suspected third escapee from the Zodiac Killer:

The first murders widely attributed to the Zodiac Killer were the shootings of high school students Betty Lou Jensen and David Faraday on December 20, 1968, on Lake Herman Road, just inside Benicia city limits. The couple were on their first date and planned to attend a Christmas concert at Hogan High School about three blocks from Jensen's home. The couple instead visited a friend before stopping at a local restaurant and then driving out on Lake Herman Road. At about 10:15 p.m., Faraday parked his mother's Rambler in a gravel turnout, which was a well-known lovers' lane. Shortly after 11:00 p.m., their bodies were found by Stella Borges, who lived nearby. The Solano County Sheriff's Department investigated the crime but no leads developed.^[13]

Utilizing available forensic data, Robert Graysmith postulated that another car pulled into the turnout, just prior to 11:00 pm and parked beside the couple. The killer apparently exited the second car and walked toward the Rambler, possibly ordering the couple out of the Rambler. Jensen appeared to have exited the car first, yet when Faraday was halfway out, the killer apparently shot Faraday in the head. Fleeing from the killer, Jensen was gunned down twenty-eight feet from the car with five shots through her back. The killer then drove off.^[14]

Just before midnight on July 4, 1969, Darlene Ferrin and Michael Mageau drove into the Blue Rock Springs Park in Vallejo, four miles (6.4 km) from the Lake Herman Road murder site, and parked. While the couple sat in Ferrin's car, a second car drove into the lot and parked alongside them but almost immediately drove away. Returning about 10 minutes later, this second car parked behind them. The driver of the second car then exited the vehicle, approaching the passenger side door of Ferrin's car, carrying a flashlight and a 9 mm Luger. The killer directed the flashlight into Mageau's and Ferrin's eyes before shooting at them, firing five times. Both victims were hit, and several bullets had passed through Mageau and into Ferrin. The killer walked away from the car but upon hearing Mageau's moaning, returned and shot each victim twice more before driving off.^[15]

On July 5, 1969, at 12:40 a.m., a man phoned the Vallejo Police Department to report and claim responsibility for the attack.

The caller also took credit for the murders of Jensen and Faraday six-and-a-half months earlier.

Police traced the call to a phone booth at a gas station at Springs Road and Tuolumne, located about three-tenths of a mile (500 m) from Ferrin's home and only a few blocks from the Vallejo Police Department.^[16] Ferrin was pronounced dead at the hospital. Mageau survived the attack despite being shot in the face, neck and chest.^[17] Mageau described his attacker as a 26-to-30-year-old, 195-to-200-pound (88 to 91 kg) or possibly even more, 5-foot-8-inch (1.73 m) white male with short, light brown curly hair.

On August 1, 1969, three letters prepared by the killer were received at the Vallejo Times Herald, the San Francisco Chronicle, and The San Francisco Examiner. The nearly identical letters—subsequently described by a psychiatrist to have been written by "someone you would expect to be brooding and isolated"^[18]—took credit for the shootings at Lake Herman Road and Blue Rock Springs. Each letter also included one-third of a 408-symbol cryptogram which the killer claimed contained his identity. The killer demanded they be printed on each paper's front page or he would "cruse [sic] around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend."^[19]

The Chronicle published its third of the cryptogram on page four of the next day's edition. An article printed alongside the code quoted Vallejo Police Chief Jack E. Stiltz as saying "We're not satisfied that the letter was written by the murderer" and requested the writer send a second letter with more facts to prove his identity.^[21] The threatened murders did not happen, and all three parts were eventually published.

On August 7, 1969, another letter was received at The San Francisco Examiner with the salutation "Dear Editor This is the Zodiac speaking." This was the first time the killer had used this name for identification. The letter was a response to Chief Stiltz's request for more details that would prove he had killed Faraday, Jensen and Ferrin. In it, the Zodiac included details about the murders which had not yet been released to the public, as well as a message to the police that when they cracked his code "they will have me."^[22]

On August 8, 1969, Donald and Bettye Harden of Salinas, California, cracked the 408-symbol cryptogram. It contained a misspelled message in which the killer said he was collecting slaves for the afterlife. No name appears in the decoded text, and the killer said that he would not give away his identity because it would slow down or stop his slave collection.^[20]

On September 27, 1969, Pacific Union College students Bryan Hartnell and Cecelia Shepard were picnicking at Lake Berryessa on a small island connected by a sand spit to Twin Oak Ridge. A white man, about 5 feet 11 inches (1.80 m) weighing more than a 170 pounds (77 kg) with combed greasy brown hair, approached them wearing a black executioner's-type hood with clip-on sunglasses over the eye-holes and a bib-like device on his chest that had a white three-by-three-inch (7.6 cm × 7.6 cm) cross-circle symbol on it. He approached them with a gun, which Hartnell believed to be a .45. The hooded man claimed to be an escaped convict from a jail with a two-word name, in either Colorado or Montana (a police officer later inferred he had been referring to a jail in Deer Lodge, Montana), where he had killed a guard and subsequently stolen a car, explaining that he now needed their car and money to go to Mexico, as the vehicle he had been driving was "too hot".^[23] He had brought precut lengths of plastic clothesline and told Shepard to tie up Hartnell, before he tied her up. The killer checked, and tightened Hartnell's bonds after discovering Shepard had bound Hartnell's hands loosely. Hartnell initially believed this event to be a weird robbery, but the man drew a knife and stabbed them both repeatedly, Hartnell suffering six and Shepard ten wounds in the process.^[18][25] The killer then hiked 500 yards (460 m) back up to Knoxville Road, drew the cross-circle symbol on Hartnell's car door with a black felt-tip pen, and wrote beneath it: "Vallejo/12-20-68/7-4-69/Sept 27–69–6:30/by knife."^[26][27]

At 7:40 p.m., the killer called the Napa County Sheriff's office from a pay telephone to report this latest crime. The caller first stated to the operator that he wished to "report a murder - no, a double murder,"^[23] before stating that he had been the perpetrator of the crime. The phone was found, still off the hook, minutes later at the Napa Car Wash on Main Street in Napa by KVON radio reporter Pat Stanley, only a few blocks from the sheriff's office, yet 27 miles (43 km) from the crime scene. Detectives were able to lift a still-wet palm print from the telephone but were never able to match it to any suspect.^[29]

After hearing their screams for help, a man and his son who were fishing in a nearby cove discovered the victims and summoned help by contacting park rangers.

Napa County Sheriff's deputies Dave Collins and Ray Land were the first law enforcement officers to arrive at the crime scene.^[30] Cecelia Shepard was conscious when Collins arrived, providing him with a detailed description of the attacker.

Hartnell and Shepard were taken to Queen of the Valley Hospital in Napa by ambulance. Shepard lapsed into a coma during transport to the hospital and never regained consciousness. She died two days later, but Hartnell survived to recount his tale to the press.^[31][32] Napa County Sheriff Detective Ken Narlow, who was assigned to the case from the outset, worked on solving the crime until his retirement from the department in 1987.^[33]

Two weeks later on October 11, 1969, a white male passenger entered the cab driven by Paul Stine at the intersection of Mason and Geary Streets (one block west from Union Square) in San Francisco requesting to be taken to Washington and Maple Streets in Presidio Heights. For reasons unknown, Stine drove one block past Maple to Cherry Street; the passenger then shot Stine once in the head with a 9mm, took Stine's wallet and car keys, and tore away a section of Stine's bloodstained shirt tail. This passenger was observed by three teenagers across the street at 9:55 p.m., who called the police while the crime was in progress. They observed a man wiping the cab down before walking away towards the Presidio, one block to the north.^[34] Two blocks from the crime scene, patrol officer Don Fouke and Eric Zelms, responding to the call, observed a white man, walking along the sidewalk east on Jackson Street, and stepping onto a stairway leading up to the front yard of one of the homes on the north side of the street; the encounter lasted only five to ten seconds.^[35]

Fouke estimated the white male pedestrian to be 35-to-45 years old, 5'10" tall and with a crew cut hair, somewhat similar but slightly older than the description of the teenagers who observed the killer in and out of Stine's cab as a 25-to-30-year-old crewcut white male at about 5-foot-8-inch (1.73 m) to 5-foot-9-inch (1.75 m) tall.

The police radio dispatcher had however initially alerted officers to be on the lookout for a black suspect, so Fouke and Zelms drove past him without stopping; the mix-up in descriptions remains unexplained.

A search ensued, but no suspects were found.

This was the last officially confirmed kill by the Zodiac Killer.

The Stine murder was initially thought to be a routine cabbie-killing, a robbery that has escalated.

However quickly, on October 13, the San Francisco Chronicle received a new letter from Zodiac containing a piece of bloody shirt and taking credit for the killing. The three teen witnesses worked with a police artist to prepare a composite sketch of Stine's killer; a few days later, this police artist returned, working with the witnesses to prepare a second composite sketch of the killer. Detectives Bill Armstrong and Dave Toschi were assigned to the case.^[1] The San Francisco Police Department investigated an estimated 2,500 suspects over a period of years.^[37]

On October 14, 1969, the Chronicle received another letter from the Zodiac, this time containing a swatch of Paul Stine's shirt tail as proof he was the killer; it also included a threat about killing schoolchildren on a school bus. To do this, Zodiac wrote, "just shoot out the front tire & then pick off the kiddies as they come bouncing out." At 2:00 p.m. on October 20, 1969, someone claiming to be the Zodiac called the Oakland Police Department (OPD), demanding that one of two prominent lawyers, F. Lee Bailey or Melvin Belli, appear on the local television show A.M. San Francisco, hosted by Jim Dunbar. Bailey was not available, but Belli did appear on the show. Dunbar appealed to the viewers to keep the lines open, and eventually, someone claiming to be the Zodiac called several times and said his name was "Sam". Belli agreed to meet with him in Daly City, but the suspect never showed up.

On November 8, 1969, the Zodiac mailed a card with another cryptogram consisting of 340 characters.

The 340-character cipher has never been decoded.^[38] Numerous possible solutions have been suggested, but none can be claimed as definitive.

On November 9, 1969, the Zodiac mailed a seven-page letter stating that two policemen stopped and actually spoke with him three minutes after he shot Stine.

Excerpts from the letter were published in the Chronicle on November 12 including the Zodiac's claim;^[39][40] that same day, Officer Don Fouke wrote a memo explaining what had happened the night of Stine's murder. On December 20, 1969, exactly one year after the murders of David Faraday and Betty Lou Jensen, the Zodiac mailed a letter to Belli that included another swatch of Stine's shirt; the Zodiac said he wanted Belli to help him.^[41]

On the night of March 22, 1970, Kathleen Johns was driving from San Bernardino to Petaluma to visit her mother. She was seven months pregnant and had her 10-month-old daughter beside her.^[42] While heading west on Highway 132 near Modesto, a car behind her began honking its horn and flashing its headlights. She pulled off the road and stopped. The man in the car parked behind her, approached her car, stated that he observed that her right rear wheel was wobbling, and offered to tighten the lug nuts. After finishing his work, the man drove off; yet when Johns pulled forward to re-enter the highway the wheel almost immediately came off the car. The man returned, offering to drive her to the nearest gas station for help. She and her daughter climbed into his car.

During the ride the car passed several service stations but the man did not stop.

For about 90 minutes he drove back and forth around the backroads near Tracy. When Johns asked why he was not stopping, he would change the subject. When the driver finally stopped at an intersection, Johns jumped out with her daughter and hid in a field. The driver searched for her using his flashlight telling her that he would not hurt her, before eventually giving up. Unable to find her, he got back into the car and drove off. Johns hitched a ride to the police station in Patterson.^[12]

When Johns gave her statement to the sergeant on duty, she noticed the police composite sketch of Paul Stine's killer and recognized him as the man who had abducted her and her child.^[43] Fearing he might come back and kill them all, the sergeant had Johns wait, in the dark, at the nearby Mil's Restaurant.

When her car was found, it had been gutted and torched.^[43]

Most accounts say he threatened to kill her and her daughter while driving them around,^[44] but at least one police report disputes that.^[12] Johns's account to Paul Avery of the Chronicle indicates her abductor left his car and searched for her in the dark with a flashlight;^[45] however, in one report she made to the police, she stated he did not leave the vehicle.^[12]

Zodiac continued to communicate with authorities for the remainder of 1970 via letters and greeting cards to the press.

In a letter postmarked April 20, 1970, the Zodiac wrote, "My name is _____," followed by a 13-character cipher.^[46] The Zodiac went on to state that he was not responsible for the recent bombing of a police station in San Francisco (referring to the February 18, 1970, death of Sgt. Brian McDonnell two days after the bombing at Park Station in Golden Gate Park)^[47] but added "there is more glory to killing a cop than a cid [sic] because a cop can shoot back." The letter included a diagram of a bomb the Zodiac claimed he would use to blow up a school bus. At the bottom of the diagram, he wrote: " = 10, SFPD = 0."^[46]

Zodiac sent a greeting card postmarked April 28, 1970, to the Chronicle. Written on the card was, "I hope you enjoy yourselves when I have my BLAST," followed by the Zodiac's cross circle signature. On the back of the card, the Zodiac threatened to use the bus bomb soon unless the newspaper published the full details he wrote. He also wanted to start seeing people wearing "some nice Zodiac butons [* sic*]."^[48]

In a letter postmarked June 26, 1970, the Zodiac stated he was upset that he did not see people wearing Zodiac buttons.

He wrote, "I shot a man sitting in a parked car with a.38."^[49] The Zodiac was possibly referring to the murder of Sgt. Richard Radetich, a week earlier, on June 19.

At 5:25 am, Radetich was writing a parking ticket in his squad car when an assailant shot him in the head with a.38-caliber pistol.

Radetich died 15 hours later.

SFPD denies the Zodiac was involved in this murder; it remains unsolved.^[47]

Included with the letter was a Phillips 66 roadmap of the San Francisco Bay Area. On the image of Mount Diablo, the Zodiac had drawn a crossed-circle similar to the ones he had included in previous correspondence. At the top of the crossed circle, he placed a zero, and then a three, six, and a nine. The accompanying instructions stated that the zero was "to be set to Mag. N."^[50] The letter also included a 32-letter cipher that the killer claimed would, in conjunction with the code, lead to the location of a bomb he had buried and set to go off in the fall. The cipher was never decoded, and the alleged bomb was never located. The killer signed the note with " = 12, SFPD = 0."

In a letter to the Chronicle postmarked July 24, 1970, the Zodiac took credit for Kathleen Johns's abduction, four months after the incident.^[51] In a July 26, 1970 letter, the Zodiac paraphrased a song from The Mikado, adding his own lyrics about making a "little list" of the ways he planned to torture his "slaves" in "paradice". The letter was signed with a large, exaggerated cross circle symbol and a new score: " = 13, SFPD = 0".^[52] A final note at the bottom of the letter stated, "P.S. The Mt. Diablo code concerns Radians + # inches along the radians."^[53] In 1981, a close examination of the radian hint by Zodiac researcher Gareth Penn led to the discovery that a radian angle, when placed over the map per Zodiac's instructions, pointed to the locations of two Zodiac attacks.^[54]

On October 7, 1970, the Chronicle received a three-by-five inch card signed by the Zodiac with the and a small cross reportedly drawn with blood. The card's message was formed by pasting words and letters from an edition of the Chronicle, and thirteen holes were punched across the card. Inspectors Armstrong and Toschi agreed it was "highly probable" the card came from the Zodiac.^[55]

On October 27, 1970, Chronicle reporter Paul Avery (who had been covering the Zodiac case) received a Halloween card signed with a letter 'Z' and the Zodiac's cross circle symbol. Handwritten on the card was the note "Peek-a-boo, you are doomed." The threat was taken seriously and received a front-page story on the Chronicle.^[7] Soon after receiving this letter, Avery received an anonymous letter alerting him to the similarities between the Zodiac's activities and the unsolved murder of Cheri Jo Bates, which had occurred four years earlier at the city college in Riverside in the Greater Los Angeles Area, more than 400 miles (640 km) south of San Francisco.^[56] He reported his findings in the Chronicle on November 16, 1970.

On October 30, 1966, 18-year-old Cheri Jo Bates, a student of Riverside Community College, spent the evening at the campus library annex until it closed at 9:00 p.m. Neighbors reported hearing a scream around 10:30 p.m. Bates was found dead the next morning, a short distance from the library, between two abandoned houses slated to be demolished for campus renovations. The wires in her Volkswagen's distributor cap had been pulled out. She was brutally beaten and stabbed to death. A man's Timex watch with a torn wristband was found nearby.^[57] The watch had stopped at 12:24,^[58] but police believe the attack occurred much earlier.^[57]

A month later, on November 29, 1966, nearly identical typewritten letters were mailed to the Riverside police and the Riverside Press-Enterprise, titled "The Confession". The author claimed responsibility for the Bates murder, providing details of the crime that were not released to the public. The author warned that Bates "is not the first and she will not be the last." ^[59] In December 1966, a poem was discovered carved into the bottom side of a desktop in the Riverside City College library. Titled "Sick of living/unwilling to die", the poem's language and handwriting resembled that of the Zodiac's letters. It was signed with what were assumed to be the initials rh. During the 1970 investigation, Sherwood Morrill, California's top "Questioned Documents" examiner, expressed his opinion that the poem was written by the Zodiac.^[60]

On April 30, 1967, exactly six months after the Bates murder, Bates' father Joseph, the Press-Enterprise, and the Riverside police all received nearly identical letters: in a handwritten scrawl the Press-Enterprise and police copies read "Bates had to die there will be more", with a small scribble at the bottom that resembled the letter Z. Joseph Bates' copy read "She had to die there will be more", this time without the Z signature.^[23]

On March 13, 1971, five months after Avery's article linking the Zodiac to the Riverside murder, the Zodiac mailed a letter to the Los Angeles Times. In the letter he credited the police, instead of Avery, for discovering his "Riverside activity, but they are only finding the easy ones, there are a hell of a lot more down there."^[62]

The connection between Cheri Jo Bates, Riverside and the Zodiac remains uncertain.

Paul Avery and the Riverside Police Department maintain that the Bates homicide was not committed by the Zodiac, but did concede some of the Bates letters may have been his work to claim credit falsely.^[63]

On March 22, 1971, a postcard to the Chronicle, addressed to "Paul Averly" and believed to be from the Zodiac, appeared to claim responsibility for the disappearance of Donna Lass on September 6, 1970.^[8] Made from a collage of advertisements and magazine lettering, it featured a scene from an advertisement for Forest Pines condominiums and the text "Sierra Club", "Sought Victim 12",^[64] "peek through the pines", "pass Lake Tahoe areas", and "around in the snow". Zodiac's cross circle symbol was in both the place of the usual return address and the lower right section of the front face of the postcard.^[23]

Lass was a nurse at the Sahara Tahoe hotel and casino. She worked until about 2:00 a.m. on September 6, 1970,^[23] treating her last patient at 1:40 a.m. Later that same day, both Lass's employer and her landlord received phone calls from an unknown male falsely claiming Lass had left town due to a family emergency.^[66] Lass was never found. What appeared to be a grave site was discovered near the Clair Tappaan Lodge in Norden, California, on Sierra Club property, but an excavation yielded only a pair of sunglasses. No evidence has been uncovered to connect the Lass disappearance with the Zodiac Killer definitively.

In a Vallejo Times-Herald story appearing on November 13, 1972,^[67] Bill Baker of the Santa Barbara County Sheriff's Office postulated that the murders of a young couple in northern Santa Barbara County might have been the work of the Zodiac Killer. On June 4, 1963, high school senior Robert Domingos and fiancée Linda Edwards were shot dead on a beach near Lompoc, having skipped school that day for "Senior Ditch Day". Police believed that the assailant attempted to bind the victims, but when they freed themselves and attempted to flee, the killer shot them repeatedly in the back and chest with a.22-caliber weapon. The killer then placed their bodies in a small shack and then tried, unsuccessfully, to burn the structure to the ground.^[68]

After the "Pines" card, the Zodiac remained silent for nearly three years.

The Chronicle then received a letter from the Zodiac, postmarked January 29, 1974, praising The Exorcist as "the best saterical comidy [* sic*] that I have ever seen". The letter included a snippet of verse from The Mikado and an unusual symbol at the bottom that has remained unexplained by researchers. Zodiac concluded the letter with a new score, "Me = 37, SFPD = 0".^[69]

Of further communications sent by the public to members of the news media, some contained similar characteristics of previous Zodiac writings.

The Chronicle received a letter postmarked February 14, 1974, informing the editor that the initials for the Symbionese Liberation Army spelled out an Old Norse word meaning "kill".^[70] However, the handwriting was not authenticated as the Zodiac's.^[71]

A letter to the Chronicle, postmarked May 8, 1974, featured a complaint that the movie Badlands was "murder-glorification" and asked the paper to cut its advertisements. Signed only "A citizen", the handwriting, tone, and surface irony were all similar to earlier Zodiac communications.^[23]*%20ISBN%C2%A00-7835-0012-2%20p]]he an anonymous letter postmarked July 8, 1974, complaining of their publishing the writings of the antifeminist columnist Marco Spinelli. The letter was signed "the Red Phantom (red with rage)". The Zodiac's authorship of this letter is debated.^[23]

A letter, dated April 24, 1978, was initially deemed authentic, but was declared a hoax less than three months later by three experts.

Dave Toschi, the SFPD homicide detective who had worked the case since the Stine murder, was thought to have forged the letter, because author Armistead Maupin believed the letter to be similar to "fan mail" he received in 1976 which he believed was authored by Toschi. While he admitted to writing the fan mail, Toschi denied forging the Zodiac letter and was eventually cleared of any charges. The authenticity of this letter remains unverified.

On March 3, 2007, an American Greetings Christmas card sent to the Chronicle, postmarked 1990 in Eureka had recently been discovered in their photo files by editorial assistant Daniel King.^[73] Inside the envelope, with the card, was a photocopy of two U.S. Postal keys on a magnet keychain. The handwriting on the envelope resembles Zodiac's print, but was declared inauthentic by forensic document examiner Lloyd Cunningham. However, not all Zodiac experts agree with Cunningham's analysis.^[74] There is no return address on the envelope nor is his crossed-circle signature to be found. The card itself is unmarked. The Chronicle turned over all the material to the Vallejo Police Department for further analysis.

In April 2004, the SFPD marked the case "inactive", citing caseload pressure and resource demands, effectively closing the case.^[75][76] However, they re-opened their case sometime before March 2007.^[4][77]

The case is open in Napa County^[78] and in the city of Riverside.^[79]

In May 2018, the Vallejo Police Department announced their intention to attempt to collect the Zodiac Killer's DNA from the back of stamps he used during his correspondence.

The analysis, by a private laboratory, is expected to utilize an advanced new technique that is able to separate DNA from the glue present on the back of stamps.^[80][81] It is hoped the Zodiac Killer may be caught in a similar fashion to the Golden State Killer. As of May 2018, a Vallejo police detective said that results were expected in several weeks.^[82][83]

Robert Graysmith's book Zodiac advanced Arthur Leigh Allen as a potential suspect based on circumstantial evidence. Allen had been interviewed by police from the early days of the Zodiac investigations and was the subject of several search warrants over a 20-year period. In 2007 Graysmith noted that several police detectives described Allen as the most likely suspect.^[84] However, in 2010, Toschi stated that all the evidence against Allen ultimately "turned out to be negative."^[85]

On October 6, 1969, Allen was interviewed by detective John Lynch of the Vallejo Police Department.

Allen had been reported in the vicinity of the Lake Berryessa attack against Hartnell and Shepard on September 27, 1969; he described himself scuba diving at Salt Point on the day of the attacks.^[86] Allen again came to police attention in 1971 when his friend Donald Cheney reported to police in Manhattan Beach, California, that Allen had spoken of his desire to kill people, use the name Zodiac, and secure a flashlight to a firearm for visibility at night. According to Cheney, this conversation occurred no later than January 1, 1969.^[87]

Jack Mulanax of the Vallejo Police Department subsequently wrote Allen had received an other than honorable discharge from the US Navy in 1958, and had been fired from his job as an elementary school teacher in March 1968 after allegations of sexual misconduct with students. He was generally well-regarded by those who knew him, but he was also described as fixated on young children and angry at women. He apparently never had a girlfriend or wife.^[88]

In September 1972, San Francisco police obtained a search warrant for Allen's residence.^[89] In 1974 Allen was arrested for committing lewd sex acts upon a 12-year-old boy;^[90] he pleaded guilty and served two years imprisonment.

Vallejo police served another search warrant at Allen's residence in February 1991.^[91] Two days after Allen's death in 1992, Vallejo police served another warrant and seized property from Allen's residence.^[92]

Other evidence existed against Allen.

A letter sent to the Riverside Police Department from Bates's killer was typed with a Royal typewriter with an Elite type, the same brand found during the February 1991 search of Allen's residence. He owned and wore a Zodiac brand wristwatch. He lived in Vallejo and worked minutes away from where one of the first victims (Ferrin) lived and from where one of the killings took place.^[37]

In 2002, SFPD developed a partial DNA profile from the saliva on stamps and envelopes of Zodiac's letters. SFPD compared this partial DNA to the DNA of Arthur Leigh Allen.^[94][95] A DNA comparison was also made with the DNA of Don Cheney, who was Allen's former close friend and the first person to suggest Allen may be the Zodiac Killer. Since neither test result indicated a match, Allen and Cheney were excluded as the contributors of the DNA, though it cannot be stated definitively that it is DNA from the Zodiac on the envelopes.^[96] However, as of March 2018, it was announced that the 2002 DNA sample was collected from outside the stamp rather than behind it, or from the envelope seal, meaning Allen could still be a suspect.^[97]

Retired police handwriting expert Lloyd Cunningham, who worked the Zodiac case for decades, added "they gave me banana boxes full of Allen's writing, and none of his writing even came close to the Zodiac.

Nor did DNA extracted from the envelopes (on the Zodiac letters) come close to Arthur Leigh Allen."^[98] While police often use document examiners during investigations, court rulings on the scientific validity of handwriting analysis have been mixed to negative.^[99]

In 2007, a man named Dennis Kaufman claimed that his stepfather Jack Tarrance was the Zodiac.^[37] Kaufman turned several items over to the FBI including a hood similar to the one worn by the Zodiac.

According to news sources, DNA analysis conducted by the FBI on the items was deemed inconclusive in 2010.^[101]

In 2009, a former lawyer named Robert Tarbox (who, in August 1975, was disbarred by the California Supreme Court for failure to pay some clients)^[102][103] said that in the early 1970s a merchant mariner walked into his office and confessed to him that he was the Zodiac Killer. The seemingly lucid seaman (whose name Tarbox would not reveal due to confidentiality) described his crimes briefly but persuasively enough to convince Tarbox. The man said he was trying to stop himself from his "opportunistic" murder spree but never returned to see Tarbox again. Tarbox took out a full-page ad in the Vallejo Times-Herald that he claimed would clear the name of Arthur Leigh Allen as a killer, his only reason for revealing the story thirty years after the fact. Robert Graysmith, the author of several books on Zodiac, said Tarbox's story was "entirely plausible".

In 2009, an episode of the History Channel television series MysteryQuest looked at newspaper editor Richard Gaikowski (1936–2004). During the time of the murders, Gaikowski worked for Good Times, a San Francisco counterculture newspaper. His appearance resembles the composite sketch, and Nancy Slover, the Vallejo police dispatcher who was contacted by the Zodiac shortly after the Blue Rock Springs Attack, has identified a recording of Gaikowski's voice as being the same as the Zodiac's.^[104]

Retired police detective Steve Hodel argues in his book The Black Dahlia Avenger that his father, George Hodel (1907–1999), was the Black Dahlia killer whose victims include Elizabeth Short.^[105] The book led to the release of previously suppressed files and wire recordings by the Los Angeles district attorney's office of his father which showed that he was a prime suspect in Short's murder. District Attorney Steve Kaye subsequently wrote a letter which is published in the revised edition stating that if George Hodel were still alive he would be prosecuted for the crimes.^[106] In a follow-up book, Hodel argued a circumstantial case that his father was also the Zodiac Killer based upon a police sketch, the similarity of the style of the Zodiac letters to the Black Dahlia Avenger letters and questioned document examination.^[107]

On February 19, 2011, America's Most Wanted featured a story about the Zodiac Killer. In 2010, a picture surfaced of known Zodiac victim Darlene Ferrin and a man who closely resembles the composite sketch, formed based on eyewitnesses' descriptions, of the Zodiac Killer. Police believe the photo was taken in San Francisco in the middle of 1966 or 1967.^[108]

Former California Highway Patrol officer Lyndon Lafferty said the Zodiac killer was a 91-year-old Solano County, California man whom he called by the pseudonym "George Russell Tucker".^[109] Using a group of retired law enforcement officers called the Mandamus Seven, Lafferty discovered "Tucker" and a cover-up for why he was not pursued.^[110] "Tucker" died in February 2012 and was not named because he was not considered a suspect by police.^[111]

In February 2014, it was reported that a man named Louis Joseph Myers had confessed to a friend in 2001 that he was the Zodiac Killer, after learning that he was dying from cirrhosis of the liver.

He requested that his friend, Randy Kenney, go to the police upon his death.

Myers died in 2002, but Kenney allegedly had difficulties getting officers to cooperate and take the claims seriously.

There are several potential connections between Myers and the Zodiac case.

Myers attended the same high schools as victims David Farraday and Betty Lou Jensen.

Myers allegedly worked in the same restaurant as victim Darlene Ferrin.

Myers also had access to the same sort of military boot whose print was found at the Lake Berryessa crime scene.

Furthermore, during the 1971–1973 period when no Zodiac letters were received, Myers was stationed overseas with the military.

Kenney says that Myers confessed he targeted couples because he had had a bad breakup with a girlfriend.

While officers associated with the case are skeptical, they believe the story is credible enough to investigate.^[112]

Robert Ivan Nichols aka Joseph Newton Chandler III was a formerly unidentified identity thief who committed suicide in Eastlake, Ohio, in July 2002. After his death, investigators were unable to locate his family and discovered that he had stolen the identity of an eight-year-old boy who was killed in a car crash in Texas in 1945. The lengths to which Nichols went to hide his identity led to speculation that he was a fugitive. In late 2016, [U.S. Marshals Service-Cleveland, Ohio] announced that forensic genealogist Dr. Colleen Fitzpatrick of Identifinders International had compared the then-unidentified man's Y-STR profile to public genetic genealogy Y-STR databases to determine his possible last name was "Nicholas". In 2017 Fitzpatrick along with Dr. Margaret Press formed the non-profit DNA Doe Project, which revisited the case by analyzing the man's autosomal DNA using the same methodology they used in identifying Marcia King and Lyle Stevik. In March 2018 the DNA Doe Project identified the man as Robert Ivan Nichols. The U.S. Marshals Service announced the identification at a press conference in Cleveland on June 21, 2018. Authorities had believed that he was a fugitive of some kind. There were many theories as to what he may have been running from, none of which were confirmed. Some internet sleuths suggested that he might have been the Zodiac killer as he resembled police sketches of the Zodiac and had lived in California, where the Zodiac operated. Another theory was that he was Steven Campbell, an engineer from Cheyenne, Wyoming, wanted for attempted murder. Authorities also considered that he could have been a German soldier or Nazi official from the Second World War that had escaped to the United States.^{[113][114][115]}

Ted Kaczynski a.k.a the Unabomber was once thought to be the Zodiac Killer.^[116]

Some investigators have noted that Ohio area serial killer Edward Edwards lived in northern California during each of the Zodiac Killer's murders in the late 1960s and would have, at the time, closely matched the Zodiac's description, although others dispute that claim. According to Edwards' daughter, there are many hints that would imply Edwards was the Zodiac Killer, such as his obsession with the well known serial killer. She has said that he would make his children watch news reports on the Zodiac Killer, and would exclaim "That's not how it happened!" during some of the reports.^[117]

The Zodiac is also suspected of being the Monster of Florence.^[118][119]