A handgun recovered from the bottom of a Swedish lake is one of the most important finds in the 20-year-old search for the killer of Prime Minister Olof Palme, investigators said Tuesday.

The unexpected discovery of the Smith & Wesson revolver could breathe new life into a stalled investigation that has failed to pin down the killer or the weapon used in the 1986 murder.

However, experts cautioned the gun may have been damaged from years under water, making it difficult to determine whether it fired the bullets that downed Palme as he walked home from a Stockholm movie theater with his wife.

Police have analyzed hundreds of guns of the type that was used to kill Palme, but investigators said the revolver found in the lake in the central province of Dalarna was particularly interesting.

Divers acting on a tip-off to Swedish tabloid Expressen found the 0.357-caliber revolver on Monday and turned it over to police. The newspaper did not reveal how it got the tip, saying it wanted to protect its source.

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"This is one of the most important weapons in the investigation," chief prosecutor Agneta Blidberg told The Associated Press after a news conference in Stockholm.

The gun was used in a post office robbery in the small Dalarna town of Mockfjard in 1983. It had been long-sought by Palme investigators because the chemical properties of a bullet fired in the robbery matched those of the bullets that killed the Social Democratic leader.

Stig Edqvist, the lead police investigator in the case, said the serial number on the gun found in the lake matched that of the so-called "Mockfjard weapon" that police had been looking for. They had unsuccessfully searched the lake seven times.

Edqvist said the gun was dumped in the lake after the 1983 robbery. However, it was unclear if the gun had been retrieved at any point, meaning it was too early to tell whether it could have been used in the Palme murder and dumped in the lake again.

"We don't know if it is possible to confirm or reject" that this is the murder weapon, he said.

The gun was sent for analysis to the National Laboratory of Forensic Science in Linkoping, 200 kilometers (125 miles) southwest of Stockholm.

Roger Calsson, head of the chemical and technical unit of the laboratory, said it was doubtful the gun would yield any solid leads because there was only a "minimal" chance that it could be tested in a shooting range.

"To be able to compare the bullets with the Palme bullets and to determine the trace (on them) you have to test fire the gun," he said. "But of course it depends on the amount of rust and as well as fat contained in the gun and how deep down in the bottom sediment it has been lying."

Other interesting finds on the revolver could include fingerprints and DNA evidence, he said, "but the chances are very small, not to say microscopic, even for fingerprints if it has been in the water for that long."

Palme was on his way home to his apartment in Stockholm's old town with his wife, Lisbet, when he was shot twice in the back. As was his custom on private occasions, he had given his bodyguards the night off. His killer escaped down a dark alley.

The only man tried for the murder, an alcoholic and drug addict named Christer Pettersson, was acquitted on appeal after police failed to produce enough evidence against him. Pettersson died in 2004.

The outspoken Palme had enemies both at home and abroad. He opposed the Vietnam War, and railed against apartheid in South Africa and against the Soviet invasion of what was then Czechoslovakia.

He tried to mediate in the Iran-Iraq war, but angered Tehran by threatening to block any secret Swedish arms sales to Iran.

Palme is the only western European head of government to have been assassinated since World War II.