The Lockerbie bombing of 1988 was a terrorist attack that went down in history as one of the deadliest in aviation history with 270 people killed, and the second deadliest attack on US civilians, behind only 9/11.
What happened over Lockerbie?
On December 21, 34 years ago, Pan Am Flight 103 from London to New York was flying over the town of Lockerbie, in south-west Scotland, when a bomb planted in the forward cargo area of the Boeing 747 exploded at 7.03am local time.
The blast at 9,450m above the ground, and 38 minutes after take-off, killed all 259 passengers and crew on board, as well as 11 people on the ground.
Of the victims, 190 were American citizens. The others were from the UK, Argentina, Belgium, Bolivia, Canada, France, Germany, Hungary, India, Ireland, Israel, Italy, Jamaica, Japan, the Philippines, South Africa, Spain, Sweden, Switzerland and Trinidad and Tobago.
Ten people remain unidentified.
Witnesses described burning debris falling from the sky, while some parts of the aircraft exploded when they hit the ground.
Wreckage from the plane created a crater 12m deep and was spread over an area of 2,175 square kilometres.
The FBI and Scottish authorities collaborated on the investigation into the attack, leading to arrest warrants for two Libyan nationals in 1991.
More than a decade after the attack, Libya's president Muammar Qaddafi, a staunch opponent of the West, handed the two suspects over to the Netherlands for trial by Scottish judges.
In January 2001, Libyan intelligence officer Abdelbaset Al Megrahi was found guilty of all 270 charges of murder against him and was sentenced to life in prison.
Al Megrahi was released in 2009 by Scottish authorities after he was diagnosed with cancer and died three years later.
He was the only person to be convicted over the bombing. The other suspect, Lamen Khalifa Fhimah was acquitted.
Who is Abu Agila Masud?
Abu Agila Mohammad Masud was thought to be the third conspirator in the attack, but was not handed over for trial in the 90s.
FBI special agent Rachel Otto testified that she reviewed a copy of an interview conducted with Mr Masud in 2017 by a Libyan law enforcement officer while the suspect was in Libyan custody.
She said the interview revealed that Mr Masud had admitted to making the bomb that downed the Pan Am flight and that he had collaborated with Al Megrahi and Mr Fhimah.
In March 2020, US authorities learnt that the interview was actually carried out in 2012. The officer who interviewed Mr Masud said he was willing to testify at a trial.
“He also admitted his involvement in other plots against citizens of the United States and other western countries,” Ms Otto said in an affidavit.
“Additionally, Masud confirmed that the bombing operation was ordered by Libyan intelligence leadership. Masud confirmed that after the operation, Qaddafi thanked him and other members of the team for their successful attack on the United States.”
Under Qaddafi's rule, Libya was designated a state sponsor of terrorism from 1979 to 2006. Qaddafi was toppled in an uprising in 2011 and later killed by an armed group.
The US claims Mr Masud worked for Libya's External Security Organisation as a “technical expert” from about 1973 to 2011, rising to the rank of colonel and conducting operations on behalf of the Libyan government.
Scottish authorities said on Sunday that Mr Masud was in US custody for his alleged role as the bomb-maker.
He was charged in 2020 for his involvement in the Lockerbie bombing by then US Attorney General William Barr, when he was still in custody of Libyan authorities.
Mr Barr at the time said the US would work “arm in arm” with Scottish counterparts to bring Mr Masud to trial.
He is expected to appear before the US District Court for the District of Columbia in the coming days.