A row over the release of the Lockerbie bomber is expected to overshadow David Cameron's first official visit to the US as Prime Minister.
Mr Cameron is due to fly into the US later, a week after four Democrat senators asked Britain to investigate the release of Abdelbaset Ali Mohmed Al Megrahi, who has terminal cancer.
Megrahi is the only man to have been convicted of the Lockerbie atrocity, which saw 270 people killed in the bombing of Pan Am Flight 103 on December 21, 1988. His release from Greenock Prison was met with anger, especially among victims' relatives and politicians in the US.
The senators want an inquiry to examine claims that oil giant BP lobbied for his release to smooth an exploration deal with Libya. An influential senate committee is also to examine the case.
The Scottish Government released Megrahi from prison - apparently on compassionate grounds - in August last year after medical advice indicated he had less than three months to live.
Daniel Kawczynski, the Tory MP who chairs the Westminster all-party group on Libya, has called for a UK public inquiry into the case, and called on Scottish Justice Secretary Kenny MacAskill to apologise for the "huge error" in deciding to let the bomber out.