He first came to police attention following the apparently random killing of Betty Lou Jensen and David Faraday on December 20, 1968 near Vallejo, California. This double homicide was followed by the murder of Darlene Ferrin and near-fatal shooting of Michael Mageau in the early morning hours of July 5, 1969 also near Vallejo. Within hours, an anonymous man called police and claimed he was the person responsible for both crimes. A masked man on September 27 stabbed Bryan Hartnell and Cecila Shepherd on the shores of Lake Beryessa, then wrote a message on Hartnell's referring to the two earlier killings. On October 11 of the same year, Paul Stine was killed with a gun while driving a taxi cab on a San Francisco street.
Between the attacks on Ferrin and Mageau and on Hartnell and Shepherd, on July 31, three area newspapers received an anonymous letter from the man responsible for these attacks, which included details that police had not released. The writer demanded that the newspapers publish a three-part cipher on the front page of their newspapers, which he had enclosed in his letters. Although professional codebreakers failed to decrypt this message, a pair of amateurs succeeded in reading the message, which had been encrypted in a homophonic cipher.
The Vallejo Times-Herald, suspicious that these letters had come from a hoaxer, asked for more unpublicized details on the first two murders, to which the serial killer responded on August 7 with a letter beginning "This is the Zodiac speaking", and supplying the details. He began all of his further letters with this phrase, and referred to himself either by that name, or with a symbol created from a circle with a cross drawn over it.
Two days after Stine's murder, the Zodiac killer sent a letter to the San Francisco Examiner with a piece of Stine's blood-stained shirt, addressed with only the paper's name and the note "Please rush to editor". The Examiner received on November 8 a greeting card and another cipher from the Zodiac Killer, with a statement that appeared to mean he had killed seven people in the months of December, July, August, September and October. He made this claim clear in a seven-page diatribe that arrived at the next day, and which included a threats of killing people with a bomb, and of killing a school bus full of children. As a result of this threat, and its repetition in later letters, school busses were staffed with armed guards for several months.
The letter of November 9 also contained the message that the killer was "changing his way of collecting" -- which came to mean that he did not claim responsibility for further murders. But he did acknowledge an earlier crime.
An anonymous tip led police to an earlier murder by the Zodiac Killer, that of Cheri Jo Bates on the Riverside Community College campus around midnight on October 30, 1966. (Riverside Community College is located outside of Los Angeles.) Research in the investigator's files uncovered four different letters sent to the police, a local newspaper, and Bates' father by the killer, as well as a poem carved into a library desktop with a ballpoint pen. In response to news reports about the earlier death, the Los Angeles Times received on March 15, 1971 a letter from the Zodiac Killer acknowledging he had killed Bates, while at the same time claiming he had killed 17 people.
The hallmark of this case were the letters, 21 in all, that the Zodiac Killer sent as late as April 24, 1978. Written in a distinctive print handwriting with misspellings, they taunted the San Francisco Police Department to catch him, sometimes offering clues of where he had buried his victims, or to his identity. Many were signed with the symbol created from a circle and cross.
The total number of the Zodiac Killer's victims is not known. Robert Graysmith lists 49 names in his book, including the eight definite victims.
The actions of the Zodiac Killer inspired several movies. Best known is Dirty Harry starring Clint Eastwood, filmed in San Francisco, and released in 1971. In the movie, the killer calls himself Scorpio, who at one point kidnaps a school bus full of children, threatening to kill all of them.