In the years since Christer Pettersson’s release, theories about Mr. Palme’s death have flourished. Many of them relate to his political credentials as an idealist who fought for perceived victims of injustice, particularly in the developing world.

At the height of the Cold War, he sought a “third way” between East and West and was criticized as tilting too much toward Moscow. He opposed the war in Vietnam and castigated apartheid in South Africa.

Indeed, Sweden became a conduit for clandestine financial support to foes of the white government in Pretoria. After the collapse of apartheid in 1990, a white former security officer, Col. Eugene de Kock, alleged that an agent of the apartheid government had murdered Mr. Palme because of his stance against racial segregation.

But that was only one of many theories.

Another hypothesis linked the killing to a shadowy arms deal with India. Others implicated Kurdish separatists in Turkey.

There were suggestions of complicity by a secretive Italian masonic lodge. Other accounts laid the blame at the door of Chilean fascists supposedly taking revenge for Mr. Palme’s opposition to the Pinochet regime.

But the latest theory, by Mr. Pettersson the journalist, published on Wednesday in the magazine Filter, claimed that the police had noted Mr. Engstrom’s presence at the crime scene but that investigators failed to follow up.

The journalist, who is no relation to the man who was jailed over the crime, also found that Mr. Engstrom had access to the same kind of weapon used to kill Mr. Palme, that he 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.