Megrahi is said to be "very ill" with prostate cancer
A US senator has written to Prime Minister Gordon Brown calling for the Lockerbie bomber to be returned to prison.
Democrat senator Charles Schumer said Abdelbaset Ali al-Megrahi was released early on the assumption he only had three months to live.
Mr Schumer questioned the severity of Megrahi's prostate cancer given that the three months had now passed.
The Scottish government stood by its decision to release Megrahi.
Mr Schumer, who represents the State of New York in the US Senate, said there was "speculation" that the severity of Megrahi's condition had been exaggerated.
In his letter to Mr Brown, he said the UK should seek the "immediate" return of Megrahi to jail.
He said: "The bottom line is Megrahi should have never been released in the first place but it would be even more outrageous if he were to be able to live a long and free life after his release.
"The victims of Pan Am Flight 103 didn't get a second chance at life and neither should Megrahi.
"Justice in this case was life in prison, no exceptions."
Megrahi, who has terminal prostate cancer, was freed from Greenock Prison on 20 August before being flown home to Tripoli, the Libyan capital.
He was serving a life sentence for the 1988 Pan Am jumbo jet bombing which claimed 270 lives and his early release was greeted with anger by American relatives of those killed, many of whom were students at Syracuse University in the State of New York.
The decision to release him was taken by Scotland's Justice Secretary Kenny MacAskill, who said at the time: "There are no fixed time limits but life expectancy of less than three months may be considered an appropriate period."
In a statement issued in response to Mr Schumer's letter on Friday, a Scottish government spokesman said Mr MacAskill's decision was based on recommendations by the parole board and prison governor, and was supported by a medical report.
The clinical assessment by the Scottish Prison Service health director was that a three-month life expectancy was "a reasonable estimate for this patient".
The spokesman said: "Mr Megrahi has terminal prostate cancer and was sent back to Libya to die.
"As Mr MacAskill said when he announced his decision, he may die sooner or may live longer, given the nature of his terminal disease."