Q+A: Why Lockerbie bomber's release caused an uproar

WASHINGTON (Reuters) - Last year's release of a Libyan convicted of the Lockerbie bombing and what role BP Plc had in his case has cast a shadow on British Prime Minister David Cameron's talks with President Barack Obama on Tuesday.

BP has drawn close scrutiny from U.S. politicians, angry over oil pollution in the Gulf of Mexico following the explosion and leak at one of the company's offshore wells.

Below are some questions and answers about the case.

WHO IS ABDEL BASSET AL-MEGRAHI?

Libyan intelligence officer Abdel Basset al-Megrahi was the only person convicted of the December 21, 1988 mid-air bombing of Pan Am flight 103 over Lockerbie, Scotland, which killed 270 people, most of them Americans.

Megrahi was found to have played a "prominent part in planning and in perpetrating" the bombing of the New York to London flight and was convicted of the murder of the 259 people aboard and 11 people on the ground killed by falling debris.

Megrahi was sentenced to life imprisonment by a special Scottish court sitting in The Netherlands in 2001, with a minimum jail term of 27 years.

Scotland released Megrahi in August, 2009 on compassionate grounds after being advised that he was suffering terminal prostate cancer and had as little as three months to live. He returned to Tripoli and is still alive nearly a year later.

HOW IS MEGRAHI'S CASE RELATED TO BP PLC?

BP has said it lobbied the British government in late 2007 over a Prisoner Transfer Agreement between Britain and Libya that could allow prisoners of either nation held by the other to be sent home to serve out their sentences.

BP said it told the British government it was "concerned about the slow progress" in negotiating the agreement. It also has said it knew this could hurt an offshore oil drilling deal that required approval by the Libyan government.

The oil company said that it lobbied the British government about the prisoner transfer agreement in late 2007, when the agreement was nearing a crucial stage of negotiations.

However, BP said it was not involved in discussions regarding the Scottish executive's decision to release Megrahi in August 2009 on compassionate grounds because of his ill health.

WHAT IS THE UK-LIBYA PRISONER TRANSFER AGREEMENT?

The agreement, which entered into force on April 29, 2009, allows for the possibility of Libyans incarcerated in Britain to serve out their sentences in Libya and vice versa.

Such a transfer requires the consent of both countries.

Britain adopted the position that whether a prisoner in Scottish custody would be transferred under the agreement was solely a decision for the authorities of Scotland, which has its own legal powers within the British political system.

There was some official discussion of excluding Megrahi from the agreement, meaning that he would not be eligible for transfer to Libya to complete his sentence.

While the Scottish government wanted him excluded, the British government ultimately did not seek such an exception for anyone convicted of the Lockerbie bombing.

A week after the Prisoner Transfer Agreement went into effect, Libya asked that Megrahi be sent home to complete his sentence, according to a British Foreign Office spokesperson.

The spokesperson, who declined to be identified, said the prisoner transfer agreement covered Libyans other than Megrahi and that there are now about 40 Libyans in prison in Britain who could be eligible for such transfers.

WHY HAS MEGRAHI'S RELEASE BECOME SUCH A FOCUS NOW?

Four U.S. senators -- New Jersey's Frank Lautenberg and Robert Menendez and New York's Kirsten Gillibrand and Charles Schumer -- have written a series of letters drawing attention to the case.

They are concerned in part because many of those killed in the Lockerbie bombing lived in New York and New Jersey. Some residents resented Megrahi's release because of his ill health and are outraged that he is still alive and out of prison.

Their action coincides with a wave of American resentment of the British oil giant after its ruptured well spilled oil into the Gulf of Mexico for nearly three months.

The senators have demanded the British government and the State Department investigate the circumstances under which Megrahi was freed on compassionate grounds.

The British government has said that it had no plans to re-examine the prisoner transfer agreement, noting that Megrahi was released on compassionate grounds because of his ill health under a separate legal process.

Cameron, who took office in May, has repeatedly made clear that he thought the decision to free Megrahi was wrong.

In a letter to the four lawmakers released on Monday, U.S. Secretary of State Hillary Clinton repeated that she and other U.S. officials consistently opposed Megrahi's release.

She said the United States was encouraging Scottish and British authorities to review the circumstances leading to his release and "to consider any new information that has come to light since."

(Editing by Patricia Wilson and David Storey)