A Libyan man has been arrested on suspicion of conspiring to murder a British policewoman who was shot dead outside the Libyan embassy in London in 1984.
Police said this was "the first significant arrest" of the investigation into the killing of 25-year-old Yvonne Fletcher, which led Britain to sever diplomatic relations with Libya.
The Libyan suspect, who is in his 50s, is also suspected of money laundering, along with two other Libyans who were arrested with him.
"Over the past 31 years we have never lost our resolve to solve this case, to bring to justice those who conspired to commit this act of murder," said Richard Walton, the head of counter terrorism at London's Metropolitan Police said on Thursday (local time).
Fletcher had been patrolling a small, peaceful demonstration outside the embassy in London's St James's Square on April 17, 1984.
Several protesters were also wounded in the shooting.
The shots were believed to have been fired from inside the building, sparking an 11-day stand-off with police.
But the killer was presumed to have left Britain among the 30 staff who were then deported under diplomatic immunity.
The killing led to Britain severing diplomatic relations with Libya until 1999 and has long been an obstacle in ties between London and Tripoli, along with the 1988 bombing of a passenger jet over the Scottish town of Lockerbie.
In 1999, Libya accepted "general responsibility" for Fletcher's death.
The overthrow of Libyan leader Muammar Gaddafi's regime in 2011 has given investigators new hope of solving the Fletcher and Lockerbie crimes.
Britain was one of the leading participants in the NATO-backed military campaign that helped topple Gaddafi.
In 2012, Libya's then prime minister Abdel Rahim al-Kib laid a wreath at the memorial to Fletcher, and Scotland Yard has sent detectives to Libya to continue their investigations into the killing.
Fletcher's family, colleagues and friends laid floral tributes at the spot where she fell on the 30th anniversary of her death last year.
At the ceremony, Fletcher's friend and colleague John Murray recalled being in the ambulance with her and telling her he would find out what happened.
"I will get justice for Yvonne Fletcher," he said.
"Bearing in mind that those were the last words she heard before she died, that's a promise I made to her and a promise I'll keep."