Swedish prosecutors said Wednesday they had closed their investigation into the 1986 murder of Prime Minister Olof Palme as their main suspect was now dead.

Palme was killed on February 28, 1986, after leaving a Stockholm cinema with his wife Lisbet to walk home, having dismissed his bodyguards for the evening. An unidentified attacker shot Palme in the back and fled, leaving the 59-year-old dying in a pool of blood on the sidewalk.

Palme, who led Sweden's Social Democrats for decades and served two periods as prime minister, was one of the architects of Scandinavia's model of a strong welfare state, and a fierce Cold War-era critic of both the United States and Soviet Union.

The failure of the police to find a killer had sparked decades of conspiracy theories that have blamed a range of forces, from the CIA and Kurdish separatists to the South African security services.

Prosecutor Krister Petersson, who has led an investigation into the case since 2017, said the killer was Stig Engstrom, a suspect long known to Swedes as "Skandia man" after the company where he worked, with offices near the scene of the shooting.

Engstrom, known to have been at the scene, was repeatedly questioned in early investigations but dismissed as a serious suspect at the time. He died in 2000 in what Swedish media reported as a suspected suicide.

A 2018 book by an investigative journalist brought to light a range of previously overlooked evidence, including time stamps showing Engstrom had left his office earlier than he had told police, in time to commit the crime.

"Because the person is dead, I cannot bring charges against him and have decided to close the investigation," Petersson said.

He did not announce any major investigative breakthroughs, and said the technical evidence was not new. He felt confident the evidence would have been sufficient to arrest Engstrom, but "would not, in itself, lead to a conviction" without more evidence that it was no longer possible to obtain.

Palme's son, Marten, told public service radio he also believed Engstrom was the killer, "but unfortunately there is no real conclusive evidence."

Engstrom's family have repeatedly dismissed accusations that he was the killer. Daily Expressen quoted his ex-wife as saying in an interview in February that he was too timid to have carried out the murder. A childhood friend, Olle Madebrink, told the paper Engstrom was "the most normal person in the world. I can't believe anything else."

(With input from Reuters and AFP)