The United Kingdom, the United States and Libya have issued a joint call for justice over the Lockerbie bombing as services were held to mark the 25th anniversary of the attack, which claimed 270 lives.
The three governments gave their "deepest condolences" to relatives of those who died when Pan Am Flight 103 blew up over the Scottish town of Lockerbie on December 21, 1988, en route from London to New York.
All 259 people on board - most of them Americans heading home - were killed as well as 11 people on the ground.
"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 over the bombing - Libyan Abdelbaset Ali Mohmet al-Megrahi, who died last year still protesting his innocence.
Ceremonies held in Lockerbie and Arlington
Scotland's leader Alex Salmond was among the mourners laying wreaths at Lockerbie's Dryfesdale Cemetery, which houses a memorial to the victims.
"On this 25-year anniversary, and as the country prepares once more to relive the harrowing events of that terrible night, 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," Mr Salmond said.
UK prime minister David Cameron paid tribute to the victims' families, saying their "fortitude and resilience" showed that terrorists would never win.
"You have shown that terrorist acts cannot crush the human spirit. That is why terrorism will never prevail," he said in a statement.
"Over the last quarter of a century much attention has been focused on the perpetrators of the atrocity.
"Today our thoughts turn to its victims and to those whose lives have been touched and changed by what happened at Lockerbie that night."
In the US, attorney-general Eric Holder and Scottish officials attended a memorial service at Arlington National Cemetery just outside Washington.
Search for perpetrators continues after fall of Gaddafi
Libya admitted responsibility for the bombing in 2003 and the regime of slain dictator Moamar Gaddafi eventually paid billions 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 regime in 2011, British and US detectives have travelled to Libya to investigate whether other perpetrators could be identified.
One man they want to question is Abdullah Senussi, Gaddafi's former intelligence chief, who is awaiting trial in Libya for war crimes allegedly committed during the 2011 uprising.
Earlier this week, Libyan justice minister Salah Marghani said he planned to allow the prosecutors to talk to Senussi.
"What we are working on is finalising the arrangements for this as much as obtaining the evidence that's available with the UK and US authorities," he told ITV news.
"We all need to know the facts."