Updated

An 83-year-old former gangster who's testifying against a one-time New England Mafia boss said Thursday that he happened to walked in on the killing of a nightclub owner more than two decades ago.

Stephen "The Rifleman" Flemmi told jurors he walked into the home of then-Mafia boss Francis "Cadillac Frank" Salemme on May 10, 1993, and saw Salemme's son strangling Steven DiSarro. Flemmi says another man, Paul Weadick, was holding DiSarro's legs and Salemme was standing nearby watching.

Flemmi, the former partner of notorious gangster James "Whitey" Bulger, said he immediately left the home when realized what was going on because he was concerned the house was under surveillance.

Flemmi is the government's star witness in the case against Salemme and Weadick. He's serving life in prison for 10 killings but admitted in court he participated in as many as 50 slayings. He's the only witness expected to testify that he saw DiSarro's killing.

Salemme's son died in 1995. Salemme and Weadick deny killing DiSarro. Defense attorneys are trying to portray Flemmi as a liar who's only trying to help himself.

Prosecutors say Salemme, who headed the New England family of La Cosa Nostra in the early 1990s, had DiSarro killed because he was worried DiSarro would cooperate with authorities. DiSarro's body wasn't found until 2016, when authorities received a tip it was buried behind a building in Providence, Rhode Island.

Salemme, now 84, and Flemmi were longtime friends and partners in crime, dating back to the gang wars in Boston in the 1960s. But the two hadn't seen each other since the 1990s before Flemmi took the stand on Wednesday, and Flemmi didn't recognize Salemme, who was sitting in the courtroom.

Flemmi's testimony is expected to resume on Friday.