Both the British and Scottish governments were facing acute pressure today over the release of the Lockerbie bomber after the most stinging criticism yet of the move.
In an unprecedented attack, FBI director Robert Mueller told the Scottish Justice Secretary Kenny MacAskill he was "outraged" at the release of Abdel Basset Ali Al-Megrahi, saying it made a mockery of the law and gave comfort to terrorists.
Former Scottish first minister Jack McConnell also critcised the decision, saying it was a "grave error of judgment" which had damaged the reputation of Scotland.
The comments came as Prime Minister Gordon Brown faced calls to explain a meeting he had with Libyan leader Colonel Muammar Gadafy when he apparently discussed the case.
Mr Brown has yet to make a public comment following the Libyan's release on Thursday, with senior ministers stressing it was a matter for the devolved Scottish government alone.
Shadow foreign secretary William Hague said the British government "urgently needs to clarify the approach that it took" to negotiations with Libya.
Megrahi, who is terminally ill with cancer, was freed by Mr MacAskill on compassionate grounds after serving eight years of a life sentence. He returned home to Libya to jubilant scenes that included people waving Scottish flags.
Former US prosecutor Mr Mueller, who played a key role in the investigation into the 1988 bombing which killed 270 people, told Mr MacAskill in a letter: "Your action in releasing Megrahi is as inexplicable as it is detrimental to the cause of justice.
"Indeed your action makes a mockery of the rule of law. Your action gives comfort to terrorists around the world."
He added that he normally did not comment on the actions of other prosecutors, but he had been forced to abandon the rule because "I am outraged at your decision, blithely defended on the grounds of 'compassion'".
The Scottish Government last night responded to the letter, saying Mr MacAskill reached his conclusions on the basis of Scotland's "due process, clear evidence, and the recommendations from the parole board and prison governor".
A spokesman said: "The US authorities indicated that although they were opposed to both prisoner transfer and compassionate release, they made it clear that they regarded compassionate release as far preferable to the transfer agreement, and Mr Mueller should be aware of that.
"Mr Mueller was involved in the Lockerbie case, and therefore has strong views, but he should also be aware that while many families have opposed Mr MacAskill's decision many others have supported it."
The spokesman said Mr MacAskill would respond to Mr Mueller's letter "in due course".
He will face questions from his peers when the Scottish Parliament is recalled a week early tomorrow. A spokesman for Mr MacAskill said he would answer "any and all" questions put to him by MSPs at Holyrood in line with the proper parliamentary process.
Opposition parties have spoken out after Col Gadafy's son Saif claimed that the decision to free Megrahi was tied to a trade agreement.
Libyan television yesterday showed pictures of Col Gadasfy himself meeting Megrahi and praising "my friend" Gordon Brown and the British government for their part in securing his freedom.
Business Secretary Lord Mandelson dismissed suggestions of a deal as "offensive".
The Foreign Office said in a statement: “No deal has been made between the UK Government and Libya in relation to Megrahi and any commercial interests in the country.”