PHILADELPHIA — The perfect crime is far easier to pull off when nobody is watching.

So on a night nearly 43 years ago, while Muhammad Ali and Joe Frazier bludgeoned each other over 15 rounds in a televised title bout viewed by millions around the world, 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....