Federal and local law enforcement agencies said a body found in the northeastern Massachusetts city of Lincoln was likely that of Stephen "Stippo" Rakes, ABC News reported.
The body was found near a path. A spokeswoman for the Middlesex County District's Office said a body had been found, but declined to provide any further information.
Police told Rakes' family his death appeared to be a suicide.
Rakes reportedly had no cellphone or wallet when he was found, and police are said to be looking for his car.
Rakes was due to testify Bulger and an associate, Stephen Flemmi, threatened Rakes' daughter and forced him to turn over his liquor store to Bulger, federal prosecutors said. The store became Bulger's headquarters.
Another Bulger associate, Kevin Weeks, testified last week that no such threat ever happened.
Steve Davis, a friend who met Rakes every day for breakfast, claimed Rakes would not have killed himself.
"He was looking forward to taking the stand," Davis said. "He told me over and over he had a big bombshell to drop."