FBI agents were made up of mainly ex-police officers, spys, or people who had connections to crime syndicates who would go undercover and infiltrate the family. However, some FBI agents accepted bribes from mobs who wanted to end a mob war, paying him to put the pressure on their rivals. During the Five Families War of 1946~1955, all of the Five Families used FBI agents to do so. Also, during the conflict, the FBI sent their trusted agents to aid traitors in the families, such as when agent Daniel Cooper was sent into the ranks of the Corleone crime family to aid the newly-recruited informant, Marty Malone, in taking down the Corleones and aiding other families in crushing them. Cooper was killed by Malone in 1953 to hopefully cover up his status as a mole, but the Corleones knew that the person who killed Cooper would be the snitch in the ranks, and they had him murdered as well.
After the war, the FBI kept on fighting and aiding organized crime, especially under the leadership of J. Edgar Hoover. In 1977, when he died, the FBI's control over the Mafia weakened, and the Mafia continued to prosper.