On December 21, 1988, Pan Am Flight 103 from London to New York explodes in midair over Lockerbie, Scotland, killing all 243 passengers and 16 crew members aboard, as well as 11 Lockerbie residents on the ground. A bomb hidden inside an audio cassette player detonated in the cargo area when the plane was at an altitude of 31,000 feet. The disaster, which became the subject of Britain’s largest criminal investigation, was believed to be an attack against the United States. One hundred eighty nine of the victims were American.

Islamic terrorists were accused of planting the bomb on the plane while it was at the airport in Frankfurt, Germany. Authorities suspected the attack was in retaliation for either the 1986 U.S. air strikes against Libya, in which leader Muammar al-Qaddafi’s young daughter was killed along with dozens of other people, or a 1988 incident, in which the U.S. mistakenly shot down an Iran Air commercial flight over the Persian Gulf, killing 290 people.

Sixteen days before the explosion over Lockerbie, the U.S. embassy in Helsinki, Finland, received a call warning that a bomb would be placed on a Pan Am flight out of Frankfurt. There is controversy over how seriously the U.S. took the threat and whether travelers should have been alerted, but officials later said that the connection between the call and the bomb was coincidental.

In 1991, following a joint investigation by the British authorities and the F.B.I., Libyan intelligence agents Abdel Basset Ali al-Megrahi and Lamen Khalifa Fhimah were indicted for murder; however, Libya refused to hand over the suspects to the U.S. Finally, in 1999, in an effort to ease United Nations sanctions against his country, Qaddafi agreed to turn over the two men to Scotland for trial in the Netherlands using Scottish law and prosecutors. In early 2001, al-Megrahi was convicted and sentenced to life in prison and Fhimah was acquitted. Over the U.S. government’s objections, Al-Megrahi was freed and returned to Libya in August 2009 after doctors determined that he had only months to live.

In 2003, Libya accepted responsibility for the bombing, but didn’t express remorse. The U.N. and U.S. lifted sanctions against Libya and Libya agreed to pay each victim’s family approximately $8 million in restitution. In 2004, Libya’s prime minister said that the deal was the “price for peace,” implying that his country only took responsibility to get the sanctions lifted, a statement that infuriated the victims’ families. Pan Am Airlines, which went bankrupt three years after the bombing, sued Libya and later received a $30 million settlement.

READ MORE: Remembering the 1988 Lockerbie Bombing