Rakes, 59, of Quincy, was found dead July 17 at approximately 1:30 p.m. along a walking trail in Lincoln.
According to the station, Rakes' car was later found in the parking lot of a McDonald's in Waltham, about seven miles away.
Sources tell the station that Rakes had no identification on him and there were no obvious signs of trauma. Police are awaiting the results of toxicology tests.
Rakes had owned a liquor store in south Boston in the 1980s. He claimed Bulger and his partner Steven Flemmi forced him to sell them his store at gunpoint. He had been attending Bulger's trial in Boston almost every day and had planned to testify until the prosecution dropped him from the witness list on July 16, the day before he was found dead.
Bulger, the former leader of South Boston's Winter Hill Gang, spent 16 years on the run, becoming one of the FBI's Ten Most Wanted before authorities captured him and his girlfriend in California in 2011. He is charged with participating in 19 murders but maintains his innocence.