Former Sen. Brian Joyce died of an overdose of a drug that is typically used to treat insomnia, but medical examiners were not able to determine whether his death was the result of an accident or a suicide.

Joyce, 56, who spent almost two decades as a state representative and senator, was awaiting trial after leaving the Senate in 2016 while under federal investigation for corruption when he was found dead at his Westport home in September.

The cause of Joyce's death was "acute pentobarbital intoxication," according to a spokesman for the state's Executive Office of Public Safety and Security, which oversees the Office of the Medical Examiner.

Pentobarbital is a barbiturate "used for the treatment of short term insomnia," according to DrugBank. The New York Times reported in 2011 that pentobarbital "is often used by veterinarians to anesthetize or euthanize animals. It has also been used in physician-assisted suicide in Oregon and in Europe."

Bristol County District Attorney Thomas Quinn's office said a toxicology test of Joyce revealed "elevated levels of Pentobarbital and a slightly elevated level of Citralopram," a prescription drug that treats depression.

"At this time, we are reviewing the autopsy report and all other attendant circumstances to determine whether further investigation is warranted," the district attorney's office said.

The manner of Joyce's death was undetermined by state medical examiners, meaning the root cause of the overdose is unknown. Investigators from the district attorney's office have previously said they do not believe that foul play was involved in Joyce's death.

Joyce had pleaded not guilty to 113 counts including racketeering, extortion, mail fraud, wire fraud, money laundering and tax evasion after federal prosecutors alleged that the former Milton senator "ran his Senate office as a criminal enterprise."

Joyce represented parts of Dorchester and Mattapan in the state Senate in the 1990s.