Prime Minister David Cameron is to meet US senators over their allegations that BP lobbied for the release of the Lockerbie bomber.
Q. What was Lockerbie?
A. On 21 December 1988 Pan Am Flight 103 from London to New York exploded 31,000ft over Lockerbie, in south west Scotland, 38 minutes after take-off from London.
The 259 people on board the Boeing 747 were killed, along with 11 people on the ground.
Q. Who is al-Megrahi and what was he convicted of?
A. Born in Tripoli in 1952, Abdelbaset Ali al-Megrahi studied in the US and visited the UK a number of times in the 1970s.
A father-of-five, he described himself as the former director of Libya's Centre for Strategic Studies.
It was a role which the FBI claimed gave him cover to act as an intelligence officer for the country.
He was indicted in November 1991 and, nearly 10 years later, stood trial under Scottish law at a special court in the Netherlands.
In January 2001 he was jailed for life after being found guilty of the murder of 270 people. His co-accused Al Amin Khalifa Fhimah was found not guilty.
Q. When was he released?
A. He was released on 20 August last year.
Q. Who authorised his release and what was their given reason?
A. The release was authorised by Scottish Justice Secretary Kenny MacAskill on compassionate grounds. Al-Megrahi has terminal prostate cancer and it was said at the time that three months was a "reasonable estimate" of how long he had left to live.
Q. What is his state of health now?
A. There have been conflicting assessments of his condition.
One newspaper report recently claimed he could live for several years but others have said he is expected to die within weeks.
Q. What are US senators asking?
A. They want an investigation into allegations that BP lobbied the UK to try to ensure al-Megrahi's release from a Scottish jail.
Q. Why is it claimed the oil firm would do this?
A. It has been alleged BP pressed for an agreement in order to help secure an offshore deal with Libya.
BP has confirmed it did press for a prisoner transfer agreement (PTA) because it was aware that a delay might have "negative consequences" for UK commercial interests.
However, it has said it did not express a view about the specific form of the agreement which was a matter for the UK and Libyan governments.
It added that it had not made representations over the al-Megrahi case, which was solely a matter for the Scottish government.
Q. Why has this come up now?
A. Senators have claimed the current Gulf of Mexico oil disaster has highlighted shortcomings at BP which make the Lockerbie claims, which have surfaced previously, more credible.
New Jersey representative Robert Menendez said it showed the firm was capable of "putting profits ahead of people".
Q. What deal was done with Libya in 2007?
A. This is at the heart of the questions being raised by US senators.
The UK government of the time has said it was carrying out a process of "normalisation of relations" with Libya.
However, it has always strenuously denied that any trade deals were ever linked to the fate of the Lockerbie bomber.
Q. What was the prisoner transfer agreement?
A. deal was ratified in 2009 encompassing four agreements, including prisoner transfer, extradition, mutual legal assistance, and civil and commercial law.
It allowed Libyan prisoners being held in a UK jail to apply to serve out their sentence in their home country.
Q. Was Megrahi part of this?
A. Yes, he was included in the deal but Scottish Justice Secretary Kenny MacAskill rejected his application and decided to release him on compassionate grounds.