Britain, Libya and the United States have issued a joint call for justice in the Lockerbie airliner bombing, just as residents gathered for memorial services on the 25th anniversary of the attack.
The three governments on Saturday offered their "deepest condolences" to relatives of the 270 people who died when Pan Am Flight 103 blew up over the Scottish town of Lockerbie on December 21, 1988.
"We want all those responsible for this most brutal act of terrorism brought to justice, and to understand why it was committed," the governments said in a statement. "We are committed to cooperate fully in order to reveal the full facts of the case."
Only one person has ever been convicted in the bombing: Libyan Abdelbaset Ali Mohmet al-Megrahi, who died last year still protesting his innocence. A second Libyan suspect was acquitted of all charges.
Mom was in shock, my brother was not saying much and I just was throwing snowballs at the sky and wondering how this could have happened.
Many of the victims on the New York-bound flight, which was downed by a suitcase bomb, were American college students flying home for Christmas.
"I was angry. I was in disbelief. Mom was in shock, my brother was not saying much and I just was throwing snowballs at the sky and wondering how this could have happened," recalled Oregon resident Whitney Davis, who lost a sister and other friends in the explosion.
Bagpipes played and wreaths were laid in the Scottish town of Lockerbie on Saturday, and mourners gathered for a moment of silence at London's Westminster Abbey.
In the US, Attorney-General Eric Holder told victims' relatives at Arlington National Cemetery that they should take comfort in their unity.
"We keep calling for change and fighting for justice on behalf of those no longer with us," Holder said.
Scottish leader Alex Salmond was among the mourners laying wreaths at Lockerbie's Dryfesdale Cemetery, which houses a memorial to the victims.
"It is important that we remember that the pain and suffering of the families and friends of those who died has endured since that winter night in 1988," Salmond said.
Libya admitted responsibility for the bombing in 2003, and the government of slain leader Muammar Gaddafi eventually paid $2.7bn in compensation to victims' families as part of a raft of measures aimed at a rapprochement with the West.
Since the fall of the Gaddafi government in 2011, British and US detectives have travelled to Libya to investigate whether other perpetrators could be identified.
Many questions remain unanswered about the attack, and the three governments said they would "welcome the visit by UK and US investigators to Libya in the near future to discuss... sharing of information and documents and access to witnesses."