According to WCVB, sources say the official cause of death of Stephen Rakes, a witness in the Whitey Bulger trial, was acute cyanide toxicity by manner of homicide.
Curran tweeted cyanide was found in Rakes' blood and gastric contents.
The Middlesex District Attorney's office has charged former Sudbury resident William Camuti in connection with Rakes' death.
Camuti, 69, was originally charged with attempted murder, misleading police, and unlawful disposition of human remains. There was no word late Thursday whether the attempted murder charge was increased to murder.
The body of Rakes, 59, was discovered at 1:30 p.m. on July 17 in a wooded area off Mill Street in Lincoln.
Officials have said Camuti poisoned Rakes over a bad business deal.
The DA's office would not say what the medical reason was.