The man responsible for the 1988 Lockerbie bombing returned home to Libya on Thursday to cheering crowds, and throngs of people waving posters of the convicted killer, who flew to his native country to die after Scotland released him from prison.

Scotland's decision to free Abdel Baset al-Megrahi outraged some relatives of the 270 people killed when the jetliner blew up over a Scottish town.

Abdel Baset al-Megrahi, 57, spent roughly 11 days behind bars for each victim in the bombing. President Barack Obama said the decision to free the terminally ill bomber on compassionate grounds was a mistake and warned Libya not to give him a hero's welcome.

But thousands were on hand to greet him warmly when his plane from Scotland touched down at a military airport in Tripoli. There was a festive atmosphere with some wearing t-shirts with al-Megrahi's picture. Others waved Libyan and Scottish flags while Libyan songs blared.

The White House declared it "deeply" regretted the Scottish decision as Abdel Baset al-Megrahi left prison and flew to Libya on an Airbus dispatched to Glasgow Airport.

Scotland's justice secretary said freeing the bomber was an expression of the Scottish people's humanity but U.S. family members of Lockerbie victims expressed outrage.

"I think it's appalling, disgusting and so sickening I can hardly find words to describe it," said Susan Cohen of Cape May Court House, New Jersey, whose 20-year-old daughter, Theodora, died in the attack. "This isn't about compassionate release. This is part of give-al-Qaddafi -what-he-wants-so-we-can-have-the-oil."

Some men outside the prison made obscene gestures as al-Megrahi's prison van drove by toward the airport.

Al-Megrahi, who had served only eight years of his life sentence, was recently given only months to live after being diagnosed with advanced prostate cancer.

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.

"Some hurts can never heal, some scars can never fade," MacAskill said. "Those who have been bereaved cannot be expected to forget, let alone forgive ... However, Mr. al-Megrahi now faces a sentence imposed by a higher power."

Al-Megrahi was convicted in 2001 of taking part in the bombing of Pan Am Flight 103 on Dec. 21, 1988. He was sentenced to life in prison. The airliner exploded over Scotland and all 259 people aboard and 11 on the ground died when it crashed into the town of Lockerbie.

The former Libyan intelligence officer was sentenced to serve a minimum of 27 years in a Scottish prison for Britain's deadliest terrorist attack. But a 2007 review of his case found grounds for an appeal of his conviction, and many in Britain believe he is innocent.

White House spokesman Robert Gibbs said Thursday the United States disagreed with the decision to free al-Megrahi.

"We continue to believe that Megrahi should serve out his sentence in Scotland," Gibbs said. "On this day, we extend our deepest sympathies to the families who live every day with the loss of their loved ones."

"I don't understand how the Scots can show compassion. It's an utter insult and utterly disgusting," said Kara Weipz, of Mount Laurel, New Jersey, whose 20-year-old brother Richard Monetti was on board Pan Am Flight 103. "It's horrible. I don't show compassion for someone who showed no remorse."

MacAskill said he stood by al-Megrahi's conviction and the sentence for "the worst terrorist atrocity ever committed on U.K. soil."

He said he ruled out sending the bomber back to Libya under a prisoner-transfer agreement, saying the U.S. victims had been given assurances that al-Megrahi would serve out his sentence in Scotland. But he said that as a prisoner given less than three months to live by doctors, al-Megrahi was eligible for compassionate release.

Later Thursday, MacAskill released a letter from al-Megrahi, appealing for mercy.

"I am a family man: first and foremost I am a son, husband, father and grandfather," al-Megrahi wrote. "I have been separated from my family as a result of what I consider and unjust conviction. I have tried to bear that with a degree of equanimity and dignity."

Compassionate release is an established feature of the British judicial system when a prisoner is near death. According to officials, there have been 30 requests for release on compassionate grounds in Scotland over the last decade, 23 of which were approved.

Al-Megrahi's return will be a landmark event in Libya and a cause for celebration. His countrymen see him as an innocent victim scapegoated by the West in a campaign to turn their country into an international pariah. Many will also view his release as a moral victory for their country.

Yet it was not immediately clear exactly how al-Megrahi would be received Thursday night. He could be taken to meet Libyan leader Muammar Al-Qaddafi or appear at an annual rally held every year on Aug. 20 for Libyans to hear a progress report from al-Qaddafi's son, Saif al-Islam Qaddafi , on projects he is working on.

However, al-Megrahi may also go directly to a hospital if he needs immediate medical care.

Qaddafi engineered a rapprochement with his former critics following the Sept. 11, 2001, terrorist attacks. He renounced terrorism, dismantled Libya's secret nuclear program, accepted his government's responsibility for the Lockerbie bombing and paid compensation to the victims' families.

Western energy companies — including Britain's BP PLC — have moved into Libya in an effort to tap the country's vast oil and gas wealth.

Qaddafi lobbied hard for the return of al-Megrahi, an issue that took on added urgency when al-Megrahi was diagnosed with cancer last year.

Freeing al-Megrahi divided the Lockerbie victims' families, with many in Britain in favor and many in the U.S. adamantly opposed.

Al-Megrahi had been a known figure in the Scottish community near his prison, receiving regular treatment at the hospital. He was visited often by his wife and children, who lived in Scotland for several years.

Briton Jim Swire, whose daughter Flora died on Flight 103, welcomed the Libyan's release, saying many questions remained about what led to the bomb that exploded in the cargo hold.

"I think he should be able to go straight home to his family and spend his last days there," Swire told the BBC. "I don't believe for a moment this man was involved in the way he was found to be involved."

Among the Lockerbie victims was John Mulroy, the AP's director of international communication, who died along with five members of his family.

The Associated Press contributed to this report.