The parents of a young police woman shot dead outside the Libyan embassy in London 20 years ago have attended a service held in her memory.
Miss Fletcher's parents are hopeful her killer will be found
Wpc Yvonne Fletcher, 25, was shot at as she policed a demonstration outside the embassy in St James's Square in 1984.
A joint murder investigation has been started by British and Libyan police.
Speaking at the service in the square, Metropolitan Police Commissioner Sir John Stevens said he was confident the killer would be found.
Sir John said he had spoken to the investigating team in Libya on Saturday morning who he described as "pretty optimistic".
He said: "We are getting full co-operation from the Libyan authorities.
"We have good reason to know who pulled the trigger as perhaps they do."
Miss Fletcher's parents, Tim and Queenie, were joined by Baroness Thatcher at the service during which a minute's silence was held.
Floral wreaths were laid at the stone which marks the spot where Miss Fletcher was shot and a police helicopter flew over the crowd in a mark of respect.
In a statement from the Fletcher family they said: "We are cautiously optimistic that recent developments may finally lead to a successful prosecution of Yvonne's murderer and closure for the family."
Wpc Fletcher was policing a demonstration when she was killed
Anti-terrorist officers from Scotland Yard travelled to the Libyan capital Tripoli at the beginning of April.
The trip was arranged by Prime Minister Tony Blair when he had talks with Libyan leader, Colonel Muammar Gaddafi, on 25 March.
Libya accepted responsibility
The shot which killed Miss Fletcher was fired from inside the embassy and Scotland Yard has long suspected that Libya knows the identity of the killer.
But he is thought to have been smuggled out of Britain after the shooting, claiming diplomatic immunity.
In 1999, Libya accepted responsibility for the murder.
But earlier this year the Libyan prime minister Shukri Ghanem told the BBC his country did not accept guilt for Miss Fletcher's death.
Scotland Yard said the joint investigation will be conducted in accordance with Libyan law.
Witnesses will be summoned by the Libyan investigating magistrate and questioned in the presence of the British members of the investigation team.