(CNN) -- A group of U.S. lawmakers have called for an investigation into whether BP may have played a role in lobbying for the release of the man convicted and sentenced to life in prison for the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland.

Abdelbeset Ali Mohmed al Megrahi, now 58, was released on compassionate grounds nearly a year ago after a doctor testified that he was dying of cancer, a decision that Scottish, British and Libyan officials have said was not linked to oil or trade.

"Reports have surfaced indicating that a 2007 oil agreement may have influenced the U.K. and Scottish governments' positions concerning Mr. Megrahi's release in 2009," wrote Democratic Sen. Frank Lautenberg of New Jersey in a letter to the Senate Committee on Foreign Relations on Monday.

Britain and Libya sparred over whether Megrahi should be included in a prisoner transfer agreement the two nations were negotiating. British officials and BP said that the oil giant's interests -- the company was seeking a huge deal to drill for oil in Libya -- were a consideration in those negotiations.

Days after Britain and Libya concluded the transfer agreement, Libya and BP signed a deal the company called "the single biggest exploration financial commitment an international energy company has ever made to Libya." But Britain and BP have denied that Megrahi's release had any part of the negotiations.

"It is a matter of public record that in late 2007 BP discussed with the UK government our concern at the slow progress in concluding a prisoner transfer agreement with Libya," BP spokesman Toby Odone said Monday. "Like many others we were aware that a delay might have negative consequences for UK commercial interests, including ratification of BP's exploration agreement.

"However, we did not express a view about the specific form of the agreement, which was a matter for the UK and Libyan governments, or make representations over the al Megrahi case, which was solely a matter for the Scottish Executive and not for the UK government."

Then-British Justice Secretary Jack Straw told The Daily Telegraph in September that trade and oil -- and BP's agreement with Libya -- were certainly part of the prisoner transfer agreement talks.

"Yes, [it was] a very big part of that," Straw told the paper. "I'm unapologetic about that. ... Libya was a rogue state. We wanted to bring it back into the fold. And yes, that included trade because trade is an essential part of it and subsequently there was the BP deal."

Straw -- and then-Prime Minister Gordon Brown -- firmly denied that Megrahi's release cleared the way for BP and Libya.

"I made it clear that for us, there was never a linkage between any other issue and the Scottish government's own decision about Megrahi's future," Brown said. "On our part, there was no conspiracy, no cover-up, no double-dealing, no

deal on oil, no attempt to instruct Scottish ministers, no private assurances by me to [Libyan leader Moammar] Gadhafi."

At the same time, however, Gadhafi's son Saif al-Islam Gadhafi told CNN that that Libya pressured the British government to include the convicted terrorist in the prisoner release agreement. Initially, he said, Britain refused to heed Libya's demands that Megrahi be included in the prisoner release agreement.

"There was no mention of Mr. Megrahi until the British said 'we are ready to sign but there should be a clause mentioning that Mr. Megrahi is excluded.' And then we said no," Gadhafi said. "We were very very angry. It's not acceptable."

Gadhafi said the agreement was that Megrahi would be released if Libya dropped its appeals in the Lockerbie case. But, Gadhafi said, Libya still believes he is innocent.

In the wake of turbulence of the rupture in April of a BP deep-water well -- a rupture that is still releasing millions of gallons of crude oil into the Gulf of Mexico -- the U.S. senators grasped onto another reason to scrutinize the troubled oil company.

"The families of the victims of Pan Am flight 103 deserve to know whether justice took a back seat to commercial interests in this case," Lautenberg said in his letter.

The bombing of the Pan Am flight killed 270 people, including 11 on the ground. Megrahi, convicted on circumstantial evidence that linked him to a tiny piece of a circuit board that was part of the bomb, served eight years of his life sentence.

BP stands to earn as much as $20 billion from its Libya deal, according to Lautenberg, and is expected to start drilling off Libya's coast in the coming months.

In a letter to the British government last week, Lautenberg was joined by Democratic Sens. Kristen Gillibrand and Charles Schumer of New York and Bob Menendez of New Jersey in calling the validity of the ailing Megrahi's prognosis into question.

"Mr. Al Megrahi is still alive and reportedly living in luxury," the letter said. "The doctor responsible for the key medical opinion has told the media that not only could Mr. Al Megrahi live another 10 years, but that the Libyan government had commissioned the doctor to make his assessment."

In response, the British Embassy in Washington posted a letter on its website from the British ambassador to Gillibrand defending Megrahi's release.

"The decision to grant Mr. Megrahi's compassionate release was made on the basis of advice from the Director of Health at the Scottish Prison Service, who drew on the advice of a number of medical experts," the ambassador said.

Brian Flynn, who lost his brother in the attack and fought vigorously to deny Megrahi's freedom, said he isn't surprised that Megrahi is still alive and believes BP was directly responsible for his release.

"You can't allow the process of justice to be corrupted by the cynical mercantilism of one company," Flynn said.

CNN's Jesse Solomon and Ross Levitt contributed to this report