As in all high profile murder cases, conspiracy theories abound. The Palme case had more than its fair share, some plausible, some with holes large enough to drive a double decker through.
Palme certainly had enemies at home and abroad. His policies were often seen as anti-nationalistic and anti-American, making him enemies on the right, especially.
Some theorise that elements of these groups conspired with the police to organise the hit and those claims are given some extra credibility by the somewhat shoddy investigation afterwards.
It is almost impossible to have a conspiracy theory without the CIA figuring somewhere and the Palme case is no exception.
It is widely claimed that the American secret services tried to recruit a young Palme, but the latter was understood to have been upset by CIA's undercover role in the International Students Conference (ISC), an organisation about which Palme cared deeply.
Additionally, conservatives were in power on both sides of the Atlantic at the time, Thatcher in the UK and Reagan in the US. As a result, Sweden's more socialist approach, backed by a determined Palme, could have been perceived as a threat serious enough the CIA could have been "encouraged" to organize and cover up a assassination, it is alleged.
Probably the most popular of all the theories is that the apartheid regime in South Africa was somehow behind the killing.
A week before the murder, Palme made an impassioned speech to the Swedish People's Parliament Against Apartheid rally in Stockholm. In September 1996, Colonel Eugene de Kock, a former South African police officer, alleged that Palme had been shot and killed ten years earlier because he “strongly opposed the apartheid regime and Sweden made substantial contributions to the ANC”.
De Kock claimed the assassin was Craig Williamson, a former police colleague and a South African spy but no proof for the allegations was ever uncovered.
Domestically, many have theorised that dark forces on the right were involved with the killing. Palme perceived as an enemy, the theory goes, due in part to his taxation policies. Although it is not generally thought that these groups were directly behind a hit, it has been suggested that they helped cover things up or made a thorough investigation difficult.
Another theory was that Palme was killed by the Kurdistan group PKK. At the time, the rising tide of immigrants from the Turkey and the Middle East was seen by many as threat to Swedish society, so a terrorist faction would also represent a convenient scapegoat.
The group however never claimed any responsibility and no evidence has been uncovered to corroborate the claims.
The Stockholm police commissioner at the time, Hans Holmer, was a vocal supporter of this theory, though it was to cost him his job after it was proved false.
However, the line of enquiry was brought up once more in 2001 when a team of Swedish police officers went to interview the Kurdish rebel leader Abdullah Acalan in a Turkish prison about the latter's allegations that a dissident Kurdish group, led by his ex-wife, murdered Palme.
Once again though the investigation proved fruitless.
Other organisations that have figured prominently in Palme conspiracies include international arms dealers, the Swedish security service Säpo, and Israel's Mossad.
Questions also remain over how forensic detectives failed to find the bullets, which were to be later uncovered very close to the body "by chance".
Throw in suggestions about Palme's wife Lisbet's apparent change of coat and her longstanding lack of support for the police, and one begins to reailse it might be possible to implicate almost anyone.
For all of the people propagating elaborate conspiracy theories about Palme's murder, the least palatable is that Christer Pettersson, a petty criminal and drug addict identified by Palme's widow, simply did it on his own.
Although he was convicted of the crime in July 1989, he was later released on appeal due to lack of evidence.
Frequently appearing on TV shows after his release until his death in 2004, Pettersson made so many contrary remarks that it was never possible to work out the validity of his claims one way or the other.
According to a documentary aired on SVT in 2006, associates of Pettersson claimed that he had confessed to them his role in Palme's murder, but claimed it was an accident and that he was looking to kill a drug dealer, dressed in similar clothing.
The show also claimed that there was a huge police presence in the vicinity but it was called off shortly before the killing. Following these revelations, the Swedish police undertook a review of the case, but once again, nothing ever came of it.
To add one final twist, shortly before he was admitted to hospital, it is claimed that Pettersson wanted to see Palme's son Mårten. Despite hopes that this could eventually amount to a confession, the meeting unfortunately never happened and Pettersson took his secret to the grave.