It was March 1965, in the early days of J. Edgar Hoover's war against the Mafia. F.B.I. agents, say Congressional investigators, eavesdropped on a conversation in the headquarters of New England's organized-crime boss, Raymond Patriarca.

Two gangsters, Joseph Barboza and Vincent Flemmi, wanted Mr. Patriarca's permission to kill a small-time hoodlum, Edward Deegan, ''as they were having a problem with him,'' according to an F.B.I. log of the conversation. ''Patriarca ultimately furnished this O.K.,'' the F.B.I. reported, and three days later Mr. Deegan turned up dead in an alley, shot six times.

It was an extraordinary situation: The Federal Bureau of Investigation had evidence ahead of time that two well-known gangsters were planning a murder and that the head of the New England Mafia was involved.

But when indictments in the case were handed down in 1967, the real killers -- who also happened to be informers for the F.B.I. -- were left alone. Four other men were tried, convicted and sentenced to death or life in prison for the murder, though they had had nothing to do with it.