LONDON — The oil giant BP faced a new furor on Thursday as it confirmed that it had lobbied the British government to conclude a prisoner-transfer agreement that the Libyan government wanted to secure the release of the only person ever convicted for the 1988 Lockerbie airliner bombing over Scotland, which killed 270 people, 189 of them Americans.

The acknowledgment came after American legislators, grappling with the controversy over the company’s disastrous Gulf of Mexico oil spill, called for an investigation into BP’s actions in the case of the freed man, Abdel Basset Ali al-Megrahi.

After an initial demand for an investigation on Wednesday by four senators from New York and New Jersey, further calls for an inquiry by the Senate Foreign Relations Committee were made on Thursday by Senators Barbara Boxer and Dianne Feinstein, both Democrats of California.

Mr. Megrahi, a former Libyan intelligence agent, was released and allowed to return to Libya in August after doctors advised the Scottish government that he was likely to die within three months of prostate cancer. But nearly a year later, he remains alive and free, though kept out of sight, in Libya’s capital, Tripoli.