Matthew Stuart died of a drug overdose, according to the results of an autopsy conducted by the state.
The report cites “acute intoxication by the combined effects of ethanol and cocaine” and ruled the man’s death an accident, said Terrel Harris, a spokesman for the Executive Office of Public Safety and Security.
Stuart was found dead in September, in a Central Square, Cambridge, homeless shelter.
In 1989, Stuart’s brother, Charles, asked him to help cover up the murder of his pregnant wife, Carol DiMaiti Stuart, in Mission Hill.
Charles Stuart, one of the most notorious crime figures in Boston’s history, shot his wife and himself on Oct. 23, 1989. His unborn baby was delivered but died days later. At the time, he blamed the act on a black man, setting off a firestorm of racial tension in the neighborhood.
Willie Bennett was arrested by Boston police on unrelated charges but was for a time considered a suspect. He was later exonerated.
Matthew Stuart eventually confessed the story to police and admitted he disposed of the murder weapon for his brother.
Two years later, he accepted a deal and pleaded guilty to charges including conspiracy, insurance fraud, and possession of a firearm, in exchange for three to five years in prison.
He had disappeared from the public spotlight for the past 20 years until his death.