A grand jury indicted William Camuti, 69, in the death of Stephen Rakes, 59, on Friday, according to the press release.
The Massachusetts Medical Examiner's Office determined in October the cause of Rakes' death was acute cyanide poisoning, according to MaryBeth Long, a spokeswoman for the Middlesex District Attorney's Office.
Camuti previously confessed to putting potassium cyanide in an iced coffee that he provided to Rakes, according to testimony by Massachusetts State Trooper Michael Banks at a hearing in Concord District Court on September 10.
In a phone call on July 16, Camuti asked Rakes to meet him about a potential investment property in Wilmington, officials said. "The defendant lured the victim to a meeting at which they were to discuss a business deal," Middlesex District Attorney Marian Ryan said in a press release. "Instead the defendant used the meeting as an occasion to serve the victim a poisoned iced coffee."
Rakes took a sip and decided the coffee tasted too bitter. Camuti then drove him around the cities of Waltham, Woburn, Burlington and Lincoln for several hours until he "succumbed to ... ingesting" the cyanide, Banks testified.
Rakes reportedly had been an extortion victim of Bulger's and learned shortly before his death that prosecutors no longer planned to call him to testify, a friend said.
The day Rakes met with Camuti, surveillance video showed Rakes leaving the federal courthouse where he regularly attended Bulger's murder trial. Rakes appeared to be wearing the same clothing in which his body was found, according to authorities.
The two men met that afternoon in a McDonald's in Waltham, where officials say Camuti bought two iced coffees, laced one with two teaspoons of potassium cyanide and gave it to Rakes. Potassium cyanide interferes with the body's ability to use oxygen, and exposure to it can be rapidly fatal, according to the Centers for Disease Control and Prevention.
Investigators said Camuti dumped Rakes' body in a wooded area, where it was found the next day.
CNN's requests for comment from Camuti's attorney were not immediately returned.
In August, Camuti pleaded not guilty to charges of attempted murder, misleading police and unlawful disposition of human remains. He is being held without bail.
Bulger, an Irish mob boss in Boston, was found guilty in August on 31 of 32 counts, including extortion, money laundering, drug dealing and weapons possession. A federal jury held Bulger responsible for the murders of 11 people.