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.
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.
The U.S. Senate Foreign Relations Committee postponed a July 29 hearing into the matter, though U.S. Sen. Robert Menendez, D-N.J., a committee member, has pressed the issue.
Menendez in a letter to Scottish First Minister Alex Salmond said he sees "no other way" to get to the bottom of the matter without a formal inquiry.
In a response letter quoted by Aberdeen's daily Press and Journal, Salmond again deflected allegations the Megrahi case was linked to BP.
"We have said repeatedly that there has never, at any point, been any contact between BP and the Scottish government in relation to Megrahi," his letter to Menendez read. "The statements we have made on this issue are entirely clear and consistent."