The identity of the Zodiac Killer is one of history’s most lasting true crime mysteries. According to the History Channel, after committing a confirmed 5 murders (although the killer claimed responsibility for somewhere around 37 murders) throughout Northern California in the 1960s and taunting law enforcement through cryptic letters about their failure to apprehend him, the murders abruptly ceased in the late 1970s.

Apart from his crimes, Zodiac is perhaps best known for the very sophisticated code he used in his communications and the terrifying symbol he chose for his calling card. So, what exactly does the Zodiac Killer’s insignia represent?

Unfortunately, Zodiac himself has never explained why he decided to use the notorious crosshairs symbol that has come to represent him. According to the Times Herald, the symbol was originally identified after Zodiac had already murdered his first two victims, 16-year-old Betty Lou Jensen and 17-year-old David Arthur Faraday, on December 20, 1968. Darlene Ferrin, 22, was later slain by him, and her friend Michael Mageau was hurt. Michael Mageau was able to identify the offender on July 4, 1969. BuzzFeed claims that Zodiac mailed duplicate versions of the same letter to the San Francisco Chronicle, The Vallejo Times Herald, and The San Francisco Examiner following these crimes.

According to ABC News, Zodiac used the crosshair symbols as a form of signature in the letters to admit guilt in the killings and to incorporate a cipher or coded message. Later, on September 27, 1969, when a couple was stabbed in Napa, the symbol was observed once more. Bryan Hartnell, the victim, described the assailant as having a crosshair emblem on his chest and wearing an executioner’s hood.