1971 FBI Burglary

Interesting story:

...burglars took a lock pick and a crowbar and broke into a Federal Bureau of Investigation office in a suburb of Philadelphia, making off with nearly every document inside. They were never caught, and the stolen documents that they mailed anonymously to newspaper reporters were the first trickle of what would become a flood of revelations about extensive spying and dirty-tricks operations by the F.B.I. against dissident groups.

Video article. And the book.