Between 1976 and 1981, FBI agent Joseph Pistone lived undercover in New York as jewel thief "Donnie Brasco" while infiltrating the Bonanno crime family. Mobsters treated him like a rising associate and eventually asked him to carry out four murders to prove his loyalty.
At least once, the situation became so dangerous that the FBI had to stage a fake killing rather than risk blowing his cover by refusing the hit. Pistone's years undercover ultimately led to hundreds of indictments and more than 100 convictions, but his assignment came close to forcing him into real murder.