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The Zodiac Killer or simply Zodiac is the pseudonym of an unidentified American serial killer who operated in Northern California from at least the late 1960s to the early 1970s. The killer originated the name in a series of taunting letters and cards sent to the San Francisco Bay Area press. The letters included four cryptograms (or ciphers). Of the four cryptograms sent, only one has been definitively solved.