Shadowy Italians. The CIA. Mysterious arms dealers. Kurdish rebels. Chilean fascists. Agents of the apartheid regime in South Africa.

The cast of suspects in the unsolved killing in February 1986 of Olof Palme, the Swedish prime minister, has never been short on intrigue and theories linking his death to an array of global conspiracies.

But, in the latest turn in a long-running saga that has baffled and embarrassed Swedish police, Thomas Pettersson, a freelance journalist based in Goteborg, has brought the case closer to home, arguing after a long investigation that the killer was a Swedish man, Stig Engstrom, first linked to the case years ago as a witness.

Pettersson said that he handed his findings in late 2017 to police, and that their investigation of Engstrom was reopened based on that material.

Read more A massacre frozen in time: Skeletons in Sweden reveal ancient attack

But as with other leads in what is sometimes described as one of the world’s longest police inquiries, the newest theory is unlikely to lead to a confession.

Engstrom died in 2000, apparently having committed suicide. But his former wife, from whom he was divorced in 1999, dismissed the idea of his involvement in the killing of the prime minister.

“It is out of question,” she told the newspaper Expressen. “He was not that kind of person, that’s for sure. He was too much of a coward. He wouldn’t harm a fly.” The Swedish news media have agreed not to identify her by name.

Palme was killed while walking home from the movies with his wife, Lisbet. He was shot from behind at point-blank range.

A known petty criminal called Christer Pettersson was jailed for life for the crime in 1989, but he won his freedom on appeal later that year. He died in 2004.

In the years since Christer Pettersson’s release, theories about 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.

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At the height of the Cold War, he sought a “third way” between East and West and was criticised 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 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 Palme’s opposition to the Pinochet regime.

But the latest theory, by Pettersson the journalist, published Wednesday in the magazine Filter, claimed that police had noted 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 Engstrom had access to the same kind of weapon used to kill Palme, that he had been active in a shooting club, that he had political and private motives for killing Palme, and that his personality matched a police profile of the likely killer.

Swedish police declined to comment on the latest theory, but said investigators were still looking at a number of tracks in the inquiry.

Engstrom’s first job was with the military. He was 52 at the time of the killing. “He had not advanced at his job,” Pettersson said in a telephone 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 recognised. To make something great of himself,” the journalist added. “He enjoyed every second of being in the media.”

At the time of the killing, investigators were focused on the suspected complicity of Kurdish militants, and Engstrom was not taken seriously, according to Pettersson.

“They thought he was just a weirdo,” Pettersson said of Engstrom. “They thought he was trying to draw attention to himself.”

The journalist said he had investigated the affair for 12 years before concluding that Engstrom was the killer.

“He has the right timing, the right clothing, he has unique information, he lied, he had close access to guns of the right type,” Pettersson said. “He has some shooting experience.”

“He was right wing and Palme unfriendly,” Pettersson added. “He had a deep political interest and a deep anti-Palme sentiment.”

Marten Palme, a son of Olof Palme, was present when his father was killed, and both he and his mother identified Christer Pettersson as the gunman at the 1989 trial. He said this week that the disclosures about Engstrom “came as a surprise.”

“I think it’s a compelling story,” Palme said in a telephone interview. “Of course, It’s a bit of a difficult situation because we all believed that Christer Pettersson was guilty. But if new evidence is brought up you have to update your beliefs.”