The Obama administration has condemned the decision to release the Lockerbie bomber and called for him to be sent back to a Scottish jail.
The US president's office said it had advised Libyan officials of its view that Abdelbaset Ali Mohmed Al Megrahi should not be free.
Barack Obama's counter-terrorism adviser John Brennan criticised the "unfortunate and inappropriate and wrong decision" yesterday, the first anniversary of the bomber's release from Greenock prison on compassionate grounds.
He said: "We've expressed our strong conviction that Al Megrahi should serve out the remainder - the entirety - of his sentence in a Scottish prison."
Meanwhile New Jersey senator Robert Menendez said that a "cloud of suspicion" hung over the decision to release Megrahi, who has terminal prostate cancer.
The bomber returned to jubilant scenes in Libya on August 20 last year where he remains alive - despite being given three months to live at the time. There did not appear to be a repeat of the celebrations in Tripoli yesterday.
Mr Menendez and three other senators have written to Scottish First Minister Alex Salmond and Prime Minister David Cameron laying out the areas where "questions linger".
The information requested in the correspondence relates to Megrahi's medical diagnosis and communications between BP and the British government.
US politicians want to investigate concerns that the bomber's release was linked to an oil deal - a suggestion that has been strongly denied by all parties involved.
Megrahi is the only person convicted of the 1988 bombing of Pan Am Flight 103 over Scotland, in which 270 people died - most of them Americans.Reuse content