The state supreme court on Thursday overturned the murder conviction of the accused trigger man in a drug-related killing, the first murder conviction to be set aside in more than a decade in Maine.

The Supreme Judicial Court unanimously ruled that the trial judge erred in instructing the jury, handing a legal victory to Marcus Asante, of Fitchburg, Massachusetts. But Asante won’t be released from custody while awaiting a new trial, said Assistant Attorney General Donald Macomber.

With the ruling, the prosecution will begin anew, and the state will petition to keep Asante detained pending trial.

“It’s a clean slate. There could be plea negotiations. There could be a new trial,” attorney Rory McNamara, who handled Asante’s appeal, said Thursday. “It’s back to square one.”

Asante contended he fired in self-defense after Douglas Morin Jr. showed a gun during a drug deal in 2016. The body of Morin, 31, of Oakfield, was found in his vehicle in Sherman. He’d been shot nine times.

Superior Court Justice Harold Stewart II ordered Asante to serve 35 years in prison for murder and robbery. Two others who were in the car at the time of shooting received lesser sentences.

The supreme court agreed with Asante that the judge erred in instructing the jurors on all of the elements of robbery, and that led to an error in the instruction on self defense on the murder count.

It was the first time a homicide conviction has been overturned since 2004, when the supreme court ordered a new trial for Brandon Thongsavanh in the fatal stabbing of a Bates College student in Lewiston.

The court ruled that a reference to a profane T-shirt Thongsavanh was wearing on the night of the killing may have prejudiced the jury.

He was convicted a second time and received the same sentence, 58 years in prison, for the death of Morgan McDuffee, the captain of the school’s lacrosse team.