Sweden ended its investigation into the murder of Prime Minister Olof Palme on Wednesday after it was revealed that the main suspect in the case is already dead.

Palme was shot dead on February 28, 1986, after leaving a movie theater in Stockholm with his wife. 

Prosecutors identified the main suspect in the case as Stig Engstrom, a Swede who opposed Palme's leftwing policies.

"Because he is dead, I can't press charges against him, and have therefore decided to close the investigation," the case's chief prosecutor Krister Petersson told reporters, adding that he died in 2000.

Engstrom, also dubbed "the Skandia man" by Swedish media, was questioned as a witness early on into the investigation since he was near the murder scene, but he was deemed to be an unreliable witness by police since he changed his story frequently.

The unsolved murder that haunted Sweden

Palme's assassination shocked the nation and sparked a massive manhunt for suspects as well as numerous conspiracy theories.

The investigation was plagued by mistakes from the outset, later turning into an unsolved case that has loomed over Sweden for decades.

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Over the years, more than 100 people have confessed to the crime. The case has been surrounded by conspiracy theories pointing the finger at various possible culprits. They have included foreign intelligence, people with right-wing sympathies within Sweden's police, and a putative lone shooter.

Petty criminal Petersson was initially convicted of the killing, but the sentence was later overturned after authorities failed to produce any technical evidence against him.

Palme was the leader of Sweden's Social Democratic Party from 1969 until his death, serving as prime minister between 1969 and 1976, and from 1982 to 1986. He was viewed by some as the architect of modern Sweden, but was viewed with suspicion in conservative circles for his anti-colonial views and criticism of the United States.

rs/rc (AP, dpa)