Baltimore prosecutors on Wednesday announced the arrest of a man in the cold case homicide of an off-duty Washington, D.C., police officer in 2017.

The officer, Sgt. Tony Anthony Mason Jr., was shot to death while sitting in a parked car with a woman he had been dating, according to police. She was also shot but survived.

The case sat unsolved for five years until detectives received a tip in early 2023 that reinvigorated their investigation and led to charges against Dion Thompson, 24, prosecutors said in a news release Wednesday. Thompson, who was 18 at the time of the shooting, is currently serving time in a federal prison on unrelated drug and gun charges.

SKELETAL REMAINS FOUND AT ILLINOIS HOME IDENTIFIED AS WOMAN MISSING SINCE 2008

An attorney representing Thompson in that case didn’t immediately respond to a request for comment Wednesday afternoon.

His charging documents in the 2017 shooting don’t include a clear statement of motive and they’re based almost entirely on the account of someone who knew Thompson but didn’t directly witness the crime. The person said Thompson admitted to shooting up a parked car because as he was leaving his friend’s grandmother’s house, he spotted a vehicle whose occupants he didn’t recognize and became paranoid, assuming they “were there to either rob him or retaliate against him for all the robberies he was committing,” according to the charging documents.

Thompson learned later from watching the news that the victim was an off-duty police officer, the witness told detectives. Thompson then drove to Philadelphia to get rid of the vehicle he was driving the night of the shooting, prosecutors allege.

The charging documents reference two other people who were allegedly involved in the shooting. One later died in a car crash. Officials said no one else has yet been charged in the case.

Mason, 40, was a 17-year veteran of Washington’s Metropolitan Police Department.

Detectives noted that he was unarmed during the attack and wasn’t wearing any clothing to identify himself as a law enforcement officer. They said extensive background checks for both Mason and his companion turned up no signs of criminal or gang activity.

“For far too long, the details surrounding Sergeant Mason’s tragic death have remained a painful mystery,” said Metropolitan Police Department Chief Pamela Smith. “While we cannot erase the pain of loss or the memories of that day, we can take solace in the fact that the person responsible is being brought to justice.”

Baltimore State’s Attorney Ivan Bates said this will be the first prosecution brought by his office’s new cold case unit.