Lockerbie bomber Abdel Ali al-Megrahi was released from a prison in Scotland Thursday, allowed to return to his native Libya to die of prostate cancer.

Scotland's justice secretary freed the terrorist on compassionate grounds, showing mercy to a dying man convicted just eight years ago of murdering 270 people in the 1988 bombing of Pan Am flight 103. Families of victims of the midair terrorist attack erupted in anger over the decision to set al-Megrahi free.

Scotland has been known to deny requests for compassionate release, but just how unusual is it for convicted criminals to get off early?

In Scotland, only 23 prisoners have been released on medical grounds in the last nine years, with seven requests denied. Statistically, that means al-Megrahi had about a 75 percent chance of securing an early release.

In England and Wales, 49 prisoners have been granted compassionate release since 2004, including Ronnie Briggs, the infamous "Great Train Robber" who was released Aug. 6 by an order from Justice Secretary Jack Straw.

Each year between 60 and 100 criminals die of natural causes while serving sentences in the UK.

In the U.S., such releases are far less common on the national level, and even impending death may not be enough to secure a prisoner's compassionate release.

The Federal Bureau of Prisons approved an average of only 21.3 motions for medical release each year between 2000 and 2008. About 24 percent of the prisoners died behind bars before their motions were ruled on.

In a federal prison population of about 200,000, just 86 inmates got their sentences reduced by the Bureau of Prisons on compassionate grounds in a 4-year period from 2004-2008 — and 23 died before a final decision was made.

Al-Megrahi, who boarded a plane to return to Libya Thursday, served just eight years of his life sentence -- or one year for every 34 people he killed.