Bagpipes played and wreaths were laid in the Scottish town of Lockerbie and mourners gathered for a moment of silence at London's Westminster Abbey, while U.S. Attorney General Eric Holder told victims' relatives at Arlington National Cemetery that they should take comfort in their unity even if time cannot erase their loss.
"We keep calling for change, and fighting for justice, on behalf of those no longer with us. We rededicate ourselves - and our nation - to the qualities that defined the men and women that we lost," Holder said.
The events marked the 25th anniversary of the explosion of Pan Am 103, a New York-bound flight that exploded over Lockerbie less than an hour after takeoff from London on Dec. 21, 1988. Many of the victims were American college students flying home for Christmas, including 35 Syracuse University students participating in study abroad programs.
The attack, caused by a bomb packed into a suitcase, killed 259 people aboard the plane, and 11 others on the ground also died.
The Arlington ceremony took place beside a cairn of 270 blocks of red Scottish sandstone, a memorial structure dedicated to the attack. Wreaths flanked the structure, the ceremonial "Taps" was played and victims' relatives recited the names of the people killed.
Former FBI director Robert Mueller, said he would never forget the haunting sight of the victims' personal belongings - a white sneaker, Christmas presents, a Syracuse sweat shirt, photographs - at a warehouse in Lockerbie when he traveled there to investigate the case as a Justice Department prosecutor.
Whitney Davis lost her younger sister Shannon, a Syracuse student, and friends in the explosion. She said she learned of the attack after returning home from Syracuse, which she also attended. There was initial hope that survivors would be found and uncertainty that the explosion was an act of terror. But the grief was immediate.
"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," said Davis, of Bend, Ore., who brought her 8-year-old daughter to the memorial in Virginia.
"It's important that she know who her aunt was and who her aunt could have been," she said.
Armen Khachaturian, of Glen Ridge, N.J., attended the service in Virginia to honor his close college friend and former Fairleigh Dickinson fraternity brother, Elia Stratis, one of the victims.
"It's indescribable what you go through when you hear something like that. It's reality that hits you square in the face, but the mind just can't process it. You can't believe it," said Khachaturian, 67, an engineer who said he was attending his first Lockerbie memorial ceremony and had recently dreamt that Stratis had somehow returned to life.
"My mind is still not processing, not accepting that he's gone," he said. "He was back after like 30 years of having been lost somewhere, being captive somewhere."
In Scotland, officials including Scottish leader Alex Salmond and relatives of victims gathered at Lockerbie's Dryfesdale Cemetery on Saturday.
"In my heart, to me this is home and there was no other place I felt I should be on this very sad and special occasion," said Jane Schultz, who lost her 20-year-old son, Thomas. "It's nice and peaceful and it's where Thomas was, so it's like coming home."
Syracuse was also holding a public memorial service in a campus chapel as well as a procession to the university's Wall of Remembrance.
One man - former Libyan intelligence official Abdel Baset al-Megrahi - was convicted of the bombing, and a second Libyan suspect was acquitted of all charges. Al-Megrahi was given a life sentence, but Scottish authorities released him on humanitarian grounds in 2009 when he was diagnosed with prostate cancer. He died in Tripoli last year.
Many questions remain unanswered about the attack, but the governments of Britain, the U.S. and Libya on Saturday issued a joint statement saying they will cooperate to reveal "the full facts" of the case.
"We are striving to further deepen our co-operation and welcome the visit by U.K. and U.S. investigators to Libya in the near future to discuss all aspects of that co-operation, including sharing of information and documents and access to witnesses," the statement read.
Hui reported from London. Associated Press writer David Caruso in New York contributed to this report.