Abdel Basset al-Megrahi walks up the stairs to a waiting jet at Glasgow airport August 20, 2009. Megrahi was sentenced to 27 years in prison in 2001 for his part in blowing up New York-bound Pan Am flight 103 in December 1988, killing 259 people on board and 11 people on the ground in Lockerbie, Scotland. [Agencies]

Moammar Gadhafi lobbied hard for the return of al-Megrahi, an issue which took on an added sense of urgency when al-Megrahi was diagnosed with prostate cancer last year. He was recently given only months to live.

Al-Megrahi's conviction was largely based on the testimony of a shopkeeper who identified him as having bought a man's shirt in his store in Malta. Scraps of the garment were later found wrapped around a timing device discovered in the wreckage of the airliner. Critics of al-Megrahi's conviction question the reliability of the store owner's evidence.

He was sentenced to serve a minimum of 27 years in a Scottish prison. But a 2007 review of his case found grounds for an appeal, and many in Britain believe he is innocent. He served only eight years.

Scottish Justice Secretary Kenny MacAskill said although al-Megrahi had not shown compassion to his victims -- many of whom were American college students flying home to New York for Christmas -- MacAskill was motivated by Scottish values to show mercy.