The details behind the mystery:
On the night of 28 February 1986, Palme was shot once in the back at close range as he walked with his wife Lisbeth along a busy street in central Stockholm.
The single bullet severed his spinal cord, killing him instantly. He was 59 years old. A second bullet grazed Lisbeth.
Several witnesses glimpsed an assailant clad in a dark jacket or coat, who fled the scene into an alley and up a flight of steps to a road above.
The murder weapon, believed to a Smith & Wesson .357 magnum revolver or similar weapon, was not recovered.
A suspect with links to right-wing groups was taken into custody 17 days after the murder but was quickly released.
The lead investigator resigned after no evidence was found in a 1987 raid on a bookshop linked to the Kurdish separatist group PKK, which had recently been named a terrorist organisation by Palme’s government.
Christer Pettersson, who had a previous murder conviction, was convicted of the crime in 1989 but freed by a higher court amid doubts over the process by which Lisbeth identified him from a police line-up. Since his acquittal, no suspects have been arrested and the unsolved murder has frustrated four lead investigators.
Swedish police visited South Africa in 1996 after a former police commander there alleged the murder had been directed by apartheid-era security forces seeking to silence critics abroad.
Bestselling Swedish crime author and journalist Stieg Larsson was working on a theory connected to the South African security apparatus until his death in 2004.
Other theories have fingered diverse groups ranging from right-wing elements in Sweden’s police to Croatian separatists of people, have been questioned and more than 130 people have confessed to the crime, which became a national obsession, with an army of amateur sleuths chasing the culprit and the £4.3m Swedish crown reward.
— Sky News Excerpts