STOCKHOLM - Mr Stieg Larsson, Swedish author of the bestselling crime trilogy Millennium, had spent a long time researching the unsolved murder of Swedish Prime Minister Olof Palme after the latter's death in 1986.

Larsson, who died in 2004 before his Millennium trilogy of novels had even been published, left behind 15 boxes of files connected to the killing of Mr Palme, The Guardian reported.

Swedish newspaper Svenska Dagbladet was given access to the files by Larsson's former partner Eva Gabrielsson, said The Guardian.

She told the Swedish newspaper that she and Mr Larsson had spent much of the year after Mr Palme's murder looking into who might be to blame, focusing on the far-right groups Larsson had tracked for years.

"Palme's death had become a priority on Stieg's agenda," the Swedish paper quoted Ms Gabrielsson as saying.

The novelist reportedly identified a Swedish ex-military officer allegedly connected to South Africa's security services as having planned the murder.

Mr Palme, who was gunned down in a Stockholm street while walking with his wife, was a vehement critic of South Africa's apartheid government and had come under fire from conservatives in Sweden and overseas for his anti-colonialist views, The Guardian reported.

It said Larsson was interviewed by Swedish police and he passed them the name of the ex-military officer Bertil Wedin, who moved to northern Cyprus shortly before Mr Palme's death and has remained there since.

"The name was written with his typewriter. It is clear it was Stieg who gave this to the Palme investigation," the newspaper quoted Ms Gabrielsson as saying.

Larsson wrote in one of his documents that one "source described him [Wedin) as a contact of the murderer, another as one of the best professional killers in Europe".

Mr Wedin was never fully interviewed by police, only talking to detectives once, briefly, on the phone, according to The Guardian.

But Sweden's deputy prosecutor-general Kerstin Skarp, who is in charge of the case, told Svenska Dagbladet that Mr Wedin "is not someone being actively probed right now".

Mr Wedin, now 73, said he welcomed a new focus in the case. "I have nothing to lose from the truth being established since, fortunately, I am not the murderer," he told Svenska Dagbladet.

The near 30-year-old crime has continued to grip Sweden. A minor criminal was convicted in 1988 but later acquitted on appeal, and the case remains a mystery.

Aside from a South Africa connection, other groups linked to the murder include Yugoslav spies, rogue Swedish security agents and Kurdish independence activists, said The Guardian.

No fewer than 130 people have confessed to the killing over the decades and the troops of amateur sleuths even gained their own Swedish slang term, privatspanare, or private scouts.

In 2012, Ms Eva Rausing, wife of the Swedish Tetra Pak heir Hans Kristian Rausing, told British police she knew the identity of the killer and the location of the murder weapon before her death from a probable drugs overdose.

But The Guardian said Larsson was more likely than many to have carried out a proper investigation. The magazine he co-founded, Expo, was based on the long-running British anti-extremist publication Searchlight, for which he also wrote. Contacts at Searchlight helped Larsson to connect Mr Wedin to the Palme case and he wrote for the British magazine about the murder, Svenska Dagbladet reported.

Mr Leif Persson, an expert on the Palme case who was for 20 years a senior criminologist for the Swedish police and is also an author of crime novels, says it is credible that Larsson, an expert on the far right, first led police to Mr Wedin.

"It would not surprise me that Stieg Larsson tipped them off on this. It's consistent with his background," he told Svenska Dagbladet. "But it doesn't solve the Palme murder."

Who is Olof Palme?

Mr Olof Palme, a Social Democrat, was Swedish prime minister between 1969 and 1976 and again between 1982 and 1986. He was a leading advocate for the working classes and Third World causes, and campaigned for an end to the war in Vietnam. He was hated by conservatives at home and abroad for his anti-colonial views and criticism of the United States and South Africa's apartheid government. Some even believed he was a spy for the former Soviet Union's KGB.

Mr Palme saw himself carrying the banner of Social Democracy through Europe at a time when the Right was only temporarily in triumph. He once said: "I know that the Thatchers and the Reagans will be out in a few years. We have to survive till then."

He believed in an open government and shunned tight security. He had two bodyguards to protect him on official functions but frequently walked unattended through the Swedish capital and went on holidays unescorted to his summer cottage on the island of Gotland.

Who is Stieg Larsson?

Stieg Larsson was a leftist and a journalist for anti-fascist and communist publications. But it was his fiction writing - published posthumously - that brought him fame. His manuscripts for a crime trilogy were discovered after his death in 2004. The trilogy, Millennium, is about the darker side of Sweden, including murder mysteries, sex trafficking scandals and a secret government unit. The first book, The Girl With The Dragon Tattoo, was published in 2005 and was made into a Hollywood movie in 2011 starring Daniel Craig and Rooney Mara. Its two sequels, The Girl Who Played with Fire and The Girl Who Kicked The Hornets' Nest, were also global bestsellers.The trilogy sold more than 75 million copies in 50 countries.

Larsson completed a huge part of his fourth novel before his death. His Swedish publisher, Norstedts, said in December 2013 that it had hired an author to work on the fourth novel. It is scheduled to be published in August 2015.

The novelist had a long-time partner Eva Gabrielsson but they were never married. He died of a heart attack at age 50 on Nov 9, 2004, after walking up seven flights of stairs to his office.

10 Things about Olof Palme's Murder

1. Swedish prime minister Olof Palme died on Feb 28, 1986, after being shot in central Stockholm. He was 59.

2. Mr Palme and his wife Lisbeth were attacked as they were walking home from a cinema at about 11.30pm local time. They were not accompanied by bodyguards.

3. Mr Palme was shot at close range in the stomach and was dead on arrival at hospital. His wife was shot in the back but survived.

4. Police said a taxi driver used his mobile radio to raise the alarm after the shooting. Two young girls in a car close to the scene of the shooting also tried to help the couple.

5. The only forensic leads left by the assassin were the two bullets fired from a revolver.

6. Christer Pettersson, a substance abuser who was convicted of manslaughter in a separate case, was convicted of murder in 1988 after having been identified as the killer by Mr Palme's wife. But he was acquitted on appeal. He died in 2004.

7. The case has spawned legions of private investigations by ordinary Swedes that a name was even coined for them - privatspanare, meaning private scouts.

8. There were many theories on the murder, ranging from his death being a carefully enacted suicide to the work of foreign spy agencies. Private investigators have also pointed the finger at an array of suspects, from Sweden's own security services to Kurdish separatists and the South African and Yugoslav secret police.

9. In 2012, Ms Eva Rausing, wife of Mr Hans Kristian Rausing, heir to Swedish food packaging business Tetra Pak, told British police she knew the identity of the killer and the location of the murder weapon before her death from a probable drug overdose.

10. More than 130 people have confessed to the killing over the decades but the case remains unsolved.

Source: BBC, The Guardian, Reuters, AFP