The Unabomber was Theodore Kaczynski, who started sending out mail bombs in 1978. In 1995, he mailed his 35,000 word manifesto to a variety of people, including former victims, demanding that it be printed verbatim by a major newspaper or else there would be more bombings. The Justice Department recommended the publication of the manifesto out of concern for public safety; the theory was that someone might recognize his writing style and reveal his identity to the proper authorities.
In fact, Kacynski's younger brother David recognized the writing style, and notified authorities, who arrested him. Kaczynski is currently serving a life sentence in ADX Florence.
More information can be found at Wikipedia: Theodore Kaczynski.