A group of US senators says a "cloud of suspicion" still hangs over the release a year ago of the man responsible for the 1988 Lockerbie bombing.
Senator Robert Menendez called on Britain and Scotland to answer a number of "outstanding questions" over the case of Abdelbasset Ali al-Megrahi.
Scottish ministers freed Megrahi, who has cancer, citing medical advice that he would likely die in three months.
The Scottish government says the decision was taken in good faith.
Following his release from a Scottish prison in August 2009, Megrahi got a hero's welcome in Tripoli.
'Alive and free'
The UK Foreign Office warned ahead of the anniversary of Megrahi's release that similar scenes would be deeply insensitive to families of the 270 people killed in 1988.
Mr Menendez said that one year on, there was "anger and frustration" in the US that Megrahi was "still very much alive and very much free".
Of those who died in the bombing, 189 were Americans.
In letters to UK Prime Minister David Cameron and Scottish First Minister Alex Salmond, the four US senators setting up an inquiry into issues around the release have reiterated what they call the "persisting uncertainty about medical, legal and diplomatic issues related to" the release.
In response to the latest letter, a spokesman for Mr Salmond said the Scottish government "have published everything we can".
The spokesman added: "The senators embarrass themselves by asking us to un-redact material which we are only prevented from publishing because the US government refused permission.
"None of the issues raised in any way alters the fact that the decisions of the Scottish government were made with total integrity, and according to the due process of Scots law."
The BBC's Matthew Price, in New York, says the senators want "more information on the medical opinions that led to the conclusion that Megrahi had just three months to live and details on communications between BP and the British government".
Their move follows an earlier decision by the Scottish government not to send officials to a hearing in Washington.
Scottish Justice Secretary Kenny MacAskill, who took the decision to release Megrahi, has said that he is prepared to meet the US senators.
He said: "What's quite clear is the people of Scotland think the decision should be made here in Scotland, by the justice secretary, and they do believe it was made in good faith without any intervention, or indeed any consideration, of political, diplomatic or economic considerations."
Mr MacAskill has always maintained the decision to release Megrahi followed due process and was in keeping with the ideals of the Scottish justice system.
US Secretary of State Hillary Clinton said in a statement on Friday that the US continued to "categorically disagree" with the decision to release Megrahi.
"As we have expressed repeatedly to Scottish authorities, we maintain that al-Megrahi should serve out the entirety of his sentence in prison in Scotland. We have and will continue to reiterate this position to the Scottish and Libyan authorities," she said.