Sen. Robert Menendez said at a hearing Wednesday that Abdel Baset al-Megrahi stated last year that he had not received chemotherapy - and that medical records released by Scotland didn't say he received that treatment.
But the senator said that information from a Scottish official closely involved with the case now says al-Megrahi did start chemotherapy in July 2009.
Menendez, a New Jersey Democrat, said the conflicting accounts raise the question of who changed government documents - and why.
The senator's announcement comes the same day as a State Department official said that a review of government records found no evidence that .
The outraged families of U.S. victims of the Pan Am Flight 103 bombing. The Senate Foreign Relations Committee is investigating whether the British-based oil company had sought his freedom to help get a $900 million exploration agreement with Libya off the ground.
In prepared testimony, Nancy McEldowney, a principal deputy assistant secretary, told lawmakers that the State Department has "not identified any materials, beyond publicly available statements and correspondence, concerning attempts by BP or other companies to influence matters" related to al-Megrahi's release.
BP has acknowledged that it had urged the British government to sign a prisoner transfer agreement with Libya, but stressed it didn't specify al-Megrahi's case.
He served eight years of a life sentence for the Dec. 21, 1988, bombing over Lockerbie, Scotland, which killed all 259 people on board, most of them Americans, and 11 people on the ground. Scotland's government released the cancer-stricken man on compassionate grounds in August 2009 and he returned to Libya, outraging people on both sides of the Atlantic. Doctors advising the Scottish government gave him three months to live, but he is still alive.
McEldowney noted that in 1998, the U.S. and U.K. wrote a letter to the United Nations secretary general, outlining an agreement for al-Megrahi and another suspect, Amin Khalifa Fhimah, to be tried before a Scottish court established in the Netherlands. Al-Megrahi was convicted but Fhimah was acquitted. The letter stated, "If found guilty, the two accused will serve their sentence in the United Kingdom."
She said that back then, the U.S. sought binding assurances that would happen, but the British countered that they couldn't legally bind the hands of future governments.
"They nonetheless assured us of their political commitment that, if convicted, al-Megrahi would remain in Scotland until the completion of his sentence," McEldowney said.
Bruce Swartz, deputy assistant attorney general, said that both the Justice and State departments stressed that al-Megrahi serve his full sentence in Scotland from the very beginning.
"This was one of the earliest issues raised by the United States in connection with the negotiations for a trial before a Scottish court in the Netherlands, and the United States continued to raise it following Megrahi's conviction and incarceration," he said in prepared testimony.
Wednesday's hearing was originally scheduled for July, but senators postponed it when they couldn't get the man they wanted to testify - outgoing BP CEO Tony Hayward. The company instead offered up a regional vice president for Europe.
In a letter to Menendez this week, Hayward reiterated that BP had no involvement in al-Megrahi's release, and that "no BP witness nor document" could shed any light on the issue.