Murder detectives in Florida believe they have solved a bizarre 27-year-old cold case in which a young woman was shot dead by a balloon-carrying clown who knocked at her door.

The killer of Marlene Warren, authorities said on Thursday, was Sheila Keen, who was having an affair with the victim’s husband at the time of the May 1990 homicide and who was later to become his second wife.

According to the Palm Beach sheriff’s detective Paige McCann, Keen, who was arrested in Virginia on Tuesday, dressed as a clown with white face paint to disguise her identity and called at the house in Wellington, where Warren was at home with her son and some of his friends.

“The clown had two balloons as well as a bouquet of flowers and went to hand Marlene those items. And as she went to receive those items, she commented: ‘How nice,’” McCann told reporters at a news conference in West Palm Beach.

“It was at that time the clown pulled out a gun and shot Marlene in the face.”

She said one of the balloons was printed with the words: “You’re the greatest!”

Detectives were initially baffled by the extraordinary circumstances of the murder, which investigators said terrified many residents of Wellington, an upmarket equestrian community 15 miles from West Palm Beach.

“How many times do you have a clown come to the door, you open the door and it shoots you?” Ric Bradshaw, the Palm Beach sheriff, said at Thursday’s press conference.

Keen and her lover, Michael Warren, were scrutinized by detectives during the original investigation, in which they allegedly determined that Keen had worn a clown’s outfit in a recent promotion for an auto parts store. But no charges were ever brought and the case lay dormant until being reopened by the sheriff department’s cold case unit in 2014.

McCann said a painstaking re-examination of existing evidence, new interviews with witnesses and fresh detective work made possible by advances in DNA technology combined to point detectives back in Keen’s direction.

The suspect, now 54, was stopped and arrested on Tuesday by police in Virginia, where Keen had lived with Michael Warren since their wedding in Las Vegas in 2002. Warren was in the car with her and allowed to leave, although McCann said “the case is still ongoing” when asked on Thursday if charges against him would follow.

David Aronberg, the Palm Beach state attorney, said Keen would be extradited to Florida to answer the murder indictment against her.

“The potential penalty in this case is life in prison, and potentially the death penalty,” he said.

McCann said the work done in 1990 laid the groundwork to Keen’s arrest this week.

“In cold cases, we have a big puzzle,” she said. “Some of the pieces are already filled in. A lot was already filled in by the thorough investigation done by the initial detectives and we just needed a few little pieces.

“We were able to do that with new technologies and DNA. We were able to complete the puzzle and that led to the indictment.”