The Zodiac Killer is the pseudonym of an American serial killer who operated in northern California from at least the late 1960s to the early 1970s. The unidentified killer would send a series of taunting letters and cards to the San Francisco Bay Area press. These letters included four now infamous ciphers. The Zodiac murdered five known victims between December 1968 and October 1969. He targeted young couples. He also murdered a male cab driver. The Zodiac himself once claimed to have murdered 37 victims, and he has been linked to several other cold cases.