Scottish and US investigators have identified two new Libyan suspects in the Lockerbie airline bombing almost 27 years ago which killed 270 people.
Pam Am flight 103 was blown up over the Scottish town of Lockerbie on Dec 21, 1988 en route from London to New York. In 2001, Libyan Abdel Basset al-Megrahi was jailed for life and remains the only person to have been convicted over the bombing.
But now, Scotland's prosecution office - the Crown Office - said another two Libyans are suspected of being involved in the attack.
Here's what you should know about the case:
What happened to Pam Am flight 103?
Pam Am flight 103 was travelling from London to New York when it was blown up over the Scottish town of Lockerbie on Dec 21, 1988. A total of 270 people died, including 259 passengers and crew, and 11 people on the ground.
US and British investigators found fragments of a circuit board and a timer, and ruled that a bomb, not mechanical failure, caused the explosion.
The attack took place two years after the United States conducted a series of air strikes against Libya that nearly killed then leader Muammar Gaddafi. The air strikes were a response to the bombing in 1986 of a Berlin disco popular with US military personnel in which three people were killed. The US blamed the Libyan government for that attack.
Some observers believed the bombing of the Pam Am flight was a form of retaliation. In 2003, Gaddafi accepted his country's responsibility for the bombing and agreed to pay US$2.7 billion (S$3.72 billion) in compensation to the victims' families. But he did not admit personally ordering the attack.
Who is Abdel Basset al-Megrahi?
The Libyan is the only person to have been convicted over the bombing. He was found guilty in 2001 and jailed for life. A second Libyan accused of involvement, Lamin Fhima, was tried but was found not guilty.
In August 2009, al-Megrahi was released from a Scottish prison on the grounds that he had been diagnosed with terminal prostate cancer and had only a few months to live. He died in Libya in 2012 still protesting his innocence. His family and some relatives of the Scottish victims believed he was wrongly convicted.
In December, Scotland's top prosecutor said no new evidence had emerged to cast doubt on al-Megrahi's conviction but attempts to track down accomplices had been hampered by the violence in Libya since Gaddafi's downfall.
Who are the two new Libyan suspects?
Scottish media named one of the two new suspects as former intelligence chief Abdullah al-Senussi, who was sentenced to death in July for crimes during the 2011 uprising against Gaddafi, who is his brother-in-law. Senussi has been in custody in Libya since 2012 and is awaiting execution.
The second person was named by Scottish media as Abu Agila Mas'ud, a bomb expert, who featured in a recent US documentary by Ken Dornstein, the brother of one of the Lockerbie victims. He is also reportedly in Libyan custody.
Scotland's prosecution service, the Crown Office, said Scotland's chief public prosecutor, Lord Advocate Frank Mulholland QC, and US Attorney General Loretta Lynch recently agreed there was a legal basis for Scottish and US investigators to treat the two Libyans as suspects in the bombing.
The lord advocate has issued an International Letter of Request to the Libyan attorney general in Tripoli identifying the pair, according to the Crown Office.
"The Lord Advocate and the US Attorney General are seeking the assistance of the Libyan judicial authorities for Scottish police officers and the FBI to interview the two named suspects in Tripoli,'' it added.
Despite this apparent breakthrough, it remains unclear whether any of the new suspects can ever stand trial in Scotland since they are currently in jail. Another factor is that the Lockerbie bombing took place nearly 30 years ago.
SOURCES: AGENCE FRANCE-PRESSE, REUTERS, BBC, GUARDIAN