Bedeviled for over 34 years by the mysterious killing of Olof Palme, the Swedish prime minister who was shot in the back by an unknown assailant on a quiet Stockholm street, Sweden’s judiciary finally made its case on Wednesday.
At a news conference in Stockholm, the prosecutor Krister Petersson said that it had been proved beyond doubt that the assailant was Stieg Engstrom, a graphic designer at an insurance company, who killed himself in 2000, at the age of 66.
But the prosecutor said that he could not rule out the possibility that Mr. Engstrom had acted as part of a larger conspiracy.
Mr. Petersson said he had reached his conclusions after an exhaustive investigation that he compared to those of the Kennedy assassination and the downing of Pan Am Flight 103 over Lockerbie, Scotland.
Mr. Pettersson, the journalist, found a link between the killer and a weapons collector, a former military man who detested Mr. Palme and his socialist ideals. The prosecutor said that in 2017, the police found a weapon at the collector’s house matching the one that could have been used in the prime minister’s killing. But officials could not establish definitively that the gun was the murder weapon.
Mr. Petersson, the prosecutor, did not name the weapons dealer, as he is not a suspect.
The mystery endured through six investigations and three commissions over the years, but Mr. Engstrom eluded suspicion though he had presented himself to the police as a witness to the killing.
Mr. Palme was killed on a cold February night in 1986 after leaving a movie theater in Stockholm with his wife, Lisbeth. The assassination shocked Sweden and evolved into one of the country’s greatest mysteries.
As the cast of suspects waxed and waned, the case spawned numerous theories linking his death to dark, global conspiracies, many of them focused on South Africa.
It was Mr. Pettersson, a freelance journalist based in Goteborg, who discovered that Mr. Engstrom had worked in a building near the theater where Mr. Palme was shot and said he had been present at the scene.
The journalist also found that Mr. Engstrom had been active in a shooting club, that he had political and private motives for killing Mr. Palme and that his personality matched a police profile of the likely killer. Mr. Engstrom was 52 at the time of the killing, and was frustrated with his lot in life.
“He had not advanced at his job,” Mr. Pettersson said in an interview. “He didn’t get the positions he felt he deserved. No family. No prospects in sight. So he was kind of a disappointed man at that point of his life. “But he also had a drive to be recognized,” Mr. Petterson added. “To make something great of himself. He enjoyed every second of being in the media.”