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10 of the world's biggest unsolved mysteries

By: Bryan Nelson on Feb. 21, 2012, 6:41 p.m.
letters from Zodiac Killer

Photo: Wikimedia Commons

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The Zodiac letters

The Zodiac letters are a series of four encrypted messages believed to have been written by the famous Zodiac Killer, a serial killer who terrorized residents of the San Francisco Bay Area in the late 1960s and early 1970s. The letters were likely written as a way to taunt journalists and police, and though one of the messages has been deciphered, the three others remain uncracked.

The identity of the Zodiac Killer also remains a mystery, though no Zodiac murders have been identified since 1970.