(CNN) -- The Scottish government was "never, at any point" lobbied by oil giant BP to release convicted Lockerbie bomber Abdelbaset Ali Mohmed al Megrahi, according to a letter sent late Wednesday by Scottish First Minister Alex Salmond to Senate Foreign Relations Committee Chairman John Kerry.
"I can say unequivocally that the Scottish government has never ... received any representations from BP in relation to al Megrahi," Salmond said. "That is to say we had no submissions or lobbying of any kind from BP, either oral or written, and, to my knowledge, the subject of al Megrahi was never raised by any BP representative to any Scottish government minister."
Salmond noted, however, that BP has "admitted that it played a role ... in encouraging the U.K. Government to conclude a prisoner transfer agreement with the Libyan government."
Salmond declined Kerry's invitation to have a representative of the Scottish government testify before the Foreign Relations Committee, though he told Kerry, D-Massachusetts, in another letter that he will answer questions in writing.
Kerry's committee has also invited BP chief executive Tony Hayward to answer questions next week about BP and the release of al Megrahi, a congressional source told CNN Wednesday.
Al Megrahi was convicted and sentenced to life in prison for the 1988 Lockerbie bombing that killed 270 people. The Scottish government released him on compassionate grounds in August after doctors said he had terminal prostate cancer and just three months to live.
Al Megrahi is still alive, and news reports in recent days have questioned whether he was as sick as depicted. Questions also have been raised about whether a deal was cut to release him to protect British business interests in Libya, possibly involving BP.
Four U.S. senators -- those from New York and New Jersey -- met with British Prime Minister David Cameron this week to urge the United Kingdom to conduct an independent investigation of al Megrahi's release. Cameron has refused to do so.
The Scottish government has repeatedly insisted that BP never lobbied to free al Megrahi.
"Part of the problem here is that everyone is saying something different," New York Sen. Kirsten Gillibrand said Thursday. "The various claims from the Scottish government raise more questions. We need greater transparency here. A full investigation would shed light on many of the most serious questions."
CNN's Alan Silverleib contributed to this report