Scottish Justice Minister Kenny MacAskill in 2009 handed Lockerbie bomber Abdelbaset al-Megrahi over to Libya because of health concerns, though a doctor said recently the Libyan could live for another 10 years.
P.J. Crowley, a spokesman for the U.S. State Department, said the decision was made against the "clear wishes" of the U.S. government and the "burden" was now on Scottish officials to explain their actions.
Both sides have released information on the release last year of Megrahi on compassionate terms. Officials in the U.S. State Department said declassified documents would show Washington in "no way" wanted Megrahi released The Wall Street Journal reports.
But a report in The Times of London during the weekend said officials at the U.S. Embassy in London wanted to make sure that if he were released, Scottish authorities would transfer Megrahi on compassionate grounds, not under the terms of a prisoner transfer agreement between the British and Libyan governments.
U.S. lawmakers are probing the matter amid concerns over the role British oil company BP had in the deal. The company denied claims it had a direct role in Megrahi's case. BP, however, is expected to begin drilling for oil in the deep waters off the Libyan coast in a matter of weeks.