(CNN)Sweden has ended a 34-year investigation into the unsolved murder of the country's then-Prime Minister Olof Palme, saying the chief suspect is dead.
Palme was gunned down as he took a late-night walk after visiting a cinema in central Stockholm with his wife, Lisbet, on February 28, 1986.
The mystery over the murder of the Social Democratic leader has gripped Sweden for three decades, prompting much speculation over the assassin and their motive, as well as producing multiple conspiracy theories.
Meanwhile, investigators have interviewed more than 10,000 people, and 134 possible suspects have confessed to the murder.
Sweden's Chief Prosecutor Krister Petersson told a news conference in the country's capital on Wednesday that he believed the lone perpetrator to be a man named Stig Engström but could do nothing more to prove it.
"As the person is deceased, I cannot bring charges against him and have decided to discontinue the investigation. In my opinion, Stig Engström is the prime suspect," he said.
"My assessment is that, after over 34 years, it is difficult to believe that any further investigation would provide us with any new details and therefore I believe we have come as far as one could expect."
Suspicion focuses on one man, whom investigators "cannot get around," he said. Engström, also known as "Skandia man" after the insurance company where he worked, died in 2000.
"To a large extent, we have been at the mercy of the police investigative work that was performed closer to the time of the crime," said Petersson.
"All in all, there are a number of circumstances that point to Engström. Had the current Palme investigation group been in charge 34 years ago, Engström would have been remanded in custody had he been unable to provide satisfactory explanations for his movements and actions. My assessment is that there would have been sufficient evidence to have him detained in custody."
False confessions, conspiracy theories
Speaking alongside Petterson, Hans Melander, head of the investigation, outlined the scale of the inquiry.
"It is by far Sweden's largest criminal investigation and is sometimes compared with the murder of JFK and [the] Lockerbie bombing. It has been ongoing since 1986 and contains 22,430 different points of interest," he said.
"Ninety-thousand people are included in the preliminary investigation, of which 40,000 are named. More than 10,000 people have been interviewed, many of them several times. More than 4,000 vehicles were investigated. And 134 people have confessed to committing the murder, including 29 directly to the police."
Analysis of the two bullets found at the scene -- one of which killed the Prime Minister, while the other injured his wife -- was carried out by laboratories in Sweden and Germany as well as by the FBI in the United States, he said.
But few traces were left on the .357 Magnum caliber metal-piercing bullets and it would be impossible now, given the passage of time, to match them to a specific weapon, Melander said.
Conspiracy theories abounded after the death of Palme, who was a prominent left-wing figure on the global scene.
One involved the PKK, or Kurdistan Workers' Party, said Melander, as that group had committed a couple of murders in Sweden at the time. About 20 people were taken in for questioning but they were released due to a lack of evidence.
Others speculated that South Africa's apartheid regime may have played a role, motivated by Palme's opposition to apartheid and support for the African National Congress.
This was "quite an interesting lead due to quite specific motives," said Melander, but unfortunately there was "no specific information" to take inquiries forward.
"We have spent a lot of time trying to map other groups, persons and events in the time prior to the assassination trying to find traces of a conspiracy ... but we haven't been able to find any support for the conspiracy theory," Petersson said.
Nonetheless, he added, "it couldn't be rejected completely that he [Engström] was part of a larger conspiracy."
Engström emerged as the main suspect in 2017 after the investigating team, which had recruited new members, reviewed all the evidence and found discrepancies between his statements and those of other witnesses, said Melander.
The 52-year-old wasn't a focal point of the investigation at the time, Petersson said, but a closer look at his background revealed that he was used to weapons, having served in the military, and belonged to a shooting club.
He was also considered by those in his circle to be "very critical of the Prime Minister" and his policies, Petersson said.
Engström told investigators he had been working late at his office in Sveavägen, the street where the assassination occurred, on the evening in question. But what he wore fitted the description of the killer given by some witnesses and his version of events did not quite stack up.
"How he acted was how we believe the murderer would have acted," Petersson said.
Conviction thrown out
At the time of the killing, many newspapers criticized authorities for bungling the investigation by not setting up roadblocks immediately and slowly cordoning off the scene. Two cabinet ministers, the national police force chief and the head of the national police intelligence agency were all forced to step down as the probe developed.
Convicted murderer, petty thief and drug addict Christer Pettersson -- no relation to the chief prosecutor -- was initially convicted of Palme's murder in 1988, in part due to testimony from the Prime Minister's wife, who witnessed the killing.
Pettersson was serving a life sentence for Palme's death when his conviction was thrown out in an appeals court in 1989 due to insufficient evidence. He died in 2004.
The discovery of a handgun in a central Stockholm lake in 2006 brought hopes of new developments in the inquiry but the case remained unsolved.
Krister Petersson, who is one of Sweden's most distinguished prosecutors, specializing in organized crime, was brought in to oversee the investigation in 2016. He has been involved in a number of high-profile cases during his career including the 2003 murder of Anna Lindh, the then-foreign minister.