(CNN) -- The Massachusetts Medical Examiner's Office officially determined that the cause of death of an alleged Whitey Bulger extortion victim was homicide by cyanide poisoning, according to MaryBeth Long, a spokeswoman for the Middlesex District Attorney's Office.
The office plans to seek additional charges against William Camuti -- who has already been charged with attempted murder, misleading police and unlawful disposition of human remains --- as a result of the official autopsy report.
Camuti, 69, confessed to putting potassium cyanide in an iced coffee that he provided to Stephen Rakes, 59, according to a testimony by a Massachusetts state trooper at hearing in Concord District Court on September 10.
"He indicated that he had purchased two iced coffees. He did in fact purchase potassium cyanide, which had been delivered and stored," said Trooper Michael Banks.
Rakes took only one sip before deciding 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.
Camuti is being held without bail for 90 days after a judge deemed that his release would not "reasonably assure the safety of any other person or community," said Long.
Camuti and Rakes were longtime business associates, and Camuti allegedly poisoned Rakes at a time when Camuti owed Rakes money.
In a phone call on July 16, Camuti asked Rakes to meet him about a potential investment property in Wilmington. The investment deal was a ruse to lure Rakes into meeting him, authorities said.
The same day, 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 Camuti allegedly bought two iced coffees, laced one with two teaspoons of potassium cyanide and gave it to Rakes, authorities said. The Centers for Disease Control and Prevention says potassium cyanide interferes with the body's ability to use oxygen, and exposure to it can be rapidly fatal.
Investigators alleged Camuti dumped Rakes' body in the wooded area, where it was found the next day.
CNN's requests for comment from Camuti's attorney were not immediately returned.
Camuti pleaded not guilty to the charges at his arraignment in August.
Rakes had been a reported extortion victim of Bulger's and had learned shortly before his death that prosecutors no longer planned to call him to testify, a friend said.
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.
CNN's Laura Ly contributed to this report.