SACRAMENTO, Calif. (Reuters) - A team of volunteer codebreakers has cracked a mysterious cipher sent more than 50 years ago to a newspaper by the San Francisco serial killer who called himself the Zodiac, the FBI said on Friday.

The Zodiac Killer - who was never caught - shot or stabbed seven people in the San Francisco Bay Area over the course of about year in 1968 and 1969, killing all but two of them. During his murderous spree, he sent a series of terrifying letters to the San Francisco Chronicle newspaper.