Washington  - Tensions continued between the United States and Britain over the release of convicted Lockerbie bomber Abdel Basset al-Megrahi as the chief US law enforcer wrote an angry letter to the Scottish justice secretary.

US relatives of the 270 killed in the 1988 aeroplane bombing said they felt "sickened" and betrayed by the decision of Scottish Justice Secretary Kenny MacAskill to release al-Megrahi because he was dying of prostate cancer.

Al-Megrahi, 57, was received with celebratory acclaim at the airport on his return home to Libya on Thursday. Along with his family he received a formal reception Friday by Libyan leader Moamer Gaddafi.

Former British prime minister Tony Blair denied in an interview with CNN that Britain had made any oil deals with Libya connected to the release of al-Megrahi. The suggestion had been made by a member of Gaddafi's family.

The director of the US Federal Bureau of Investigation, Robert Mueller, charged that Scotland had shown the "back of your hand" to the relatives of those on the plane and the 11 on the ground who were killed.

The release had made "a mockery of the rule of law" and given "comfort to terrorists around the world," Mueller wrote in a letter released Saturday by the FBI.

Mueller is a former prosecutor who played a key role in the investigation.

"Your action in releasing Megrahi is as inexplicable as it is detrimental to the cause of justice," Mueller wrote.

The Scottish government, which has jurisdiction in the case, said in a statement that its justice secretary's decision was based on Scotland's "due process, clear evidence, and the recommendations from the parole board and prison governor."

Mueller charged that MacAskill had hid "behind opaque references to 'the need for compassion'," while the Scottish government pointed out that "compassionate release is not part of the US justice system but it is part of Scotland's."

In interviews with CNN, relatives expressed their dismay over the release.

"I am beyond sickened. I truly don't understand how (MacAskill) could have made this decision and how he can sleep at night," said Stephanie Bernstein, whose husband was killed in the bombing.

The former British prime minister defended his successor Gordon Brown, saying that neither man had influence over the Scottish executive in the British system.

He also said it was wrong to suggest that the release was connected to any British oil or business interests.

"The idea that the British government and the Libyans would sit down and somehow barter about the freedom of this Libyan prisoner to form some sort of business deal ... It's not only wrong, it's completely implausible," Blair said.

Mueller's letter increased the level of official US criticism. Earlier this week, US President Barack Obama called the release a "mistake" and US Secretary of State Hillary Rodham Clinton said she was deeply disappointed.

Clinton said that she and other US officials lobbied for months to prevent al-Megrahi's release, arguing he is a convicted terrorist who should be required to serve out the life imprisonment sentence handed down following his 2001 trial. (dpa)