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10 of the world's biggest unsolved mysteries

By: Bryan Nelson on Sept. 13, 2017, 10:13 a.m.
Zodiac letter sent to the San Francisco Chronicle on June 26, 1970.

Photo: WikiSource

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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, like the cipher at the bottom of this letter, remain uncracked.

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