British police on Thursday announced "the first significant arrest" in the 1984 killing of London policewoman Yvonne Fletcher, shot dead from inside the Libyan embassy in an incident that solidified Libya's reputation as a rogue state.

Commander Richard Walton, head of Scotland Yard's counter-terrorism unit, said a Libyan man in his 50s was arrested Thursday morning on suspicion of conspiracy to murder. A man and a woman, also Libyans, were arrested on a lesser charge of money-laundering in relation to the Fletcher case.

"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," Walton said.

Fletcher was killed and 10 others wounded on April 17, 1984, when someone opened fire with a submachine gun from inside the Libyan People's Bureau on St. James' Square in central London.

Fletcher was policing a large demonstration opposed to Libyan dictator Muammar Qaddafi outside the embassy, while Qaddafi supporters held a competing rally nearby. Police said the pro-Qaddafi crowd was organized by the embassy.

Fletcher was shot in the back when shots rang out from inside the embassy. A 10-day siege followed the shooting before 30 people holed up inside were deported to Libya.

The shooting led to a 15-year rupture in diplomatic relations between Britain and Libya and was a factor in Britain's support for a U.S. bombing raid on Libya ordered by President Ronald Reagan two years later.

Britain and Libya restored ties in 1999 after Libya accepted responsibility for Fletcher's shooting, apologized and agreed to pay her family compensation.

Qaddafi said in a 2009 interview that he regretted the shooting but did not know who was to blame.

"She is not an enemy to us, and we are sorry all the time and our sympathy, because she was on duty, she was there to protect the Libyan Embassy," Qaddafi said.

Police said in a statement that the incident was orchestrated from Libya as part of a campaign to attack dissidents overseas. The statement said the fall of Qaddafi's regime and his death in 2011 "provided access to new lines of inquiry" that aided the 31-year-old investigation.

Scotland Yard said new information had come to light that helped investigators better understand the conspiracy.

Police are offering a reward of up to $76,000 for information leading to those responsible for the killing and are releasing footage from the demonstration moments before the shooting started.

Walton said police particularly want to talk to people who were part of the pro-Qaddafi demonstration, including those who may be living in Libya or in other parts of the world.

"Allegiances change and we hope with the passage of time, witnesses who have not spoken to us will examine their consciences and come forward," he said.

Fletcher's family also released a statement asking anyone with information to contact police.

The three suspects have not been charged or identified. Police said they are in custody and searches are continuing.