Washington (CNN) -- An investigation into the release of convicted Pan Am Flight 103 bomber Abdelbeset al Megrahi has found the medical prognosis used to justify his release from a Scottish prison "was inaccurate and unsupported by medical science," according to a U.S. Senate report released Tuesday.
The release of the report, led by Sen. Robert Menendez, D-New Jersey, coincided with the 22nd anniversary of the bombing, which occurred over Lockerbie, Scotland, in December 1988, killing 259 people aboard the Boeing 747 and 11 on the ground.
Al Megrahi was released from a Scottish prison last year on the grounds that he had cancer and was not likely to live more than three more months -- a prognosis Menendez has questioned.
The report, titled "Justice Undone: The Release of the Lockerbie Bomber," was the result of a five-month investigation led by Menendez's office and was co-signed by Sens. Frank Lautenberg, D-New Jersey, Kirsten Gillibrand, D-New York, and Charles Schumer, D-New York.
It is based on interviews with officials from the United Kingdom, Scotland and the United States, including "substantive experts" within the U.S. State and Justice departments. Also interviewed were U.S. and British prostate cancer specialists, "academic experts on Libya and energy policy, and business executives with operations in Libya," the report says.
"The three-month prognosis given to al Megrahi by Scottish doctors was inaccurate and unsupported by medical science," the report said.
The report also took issue with the Scottish government's decision to grant al Megrahi compassionate release, indicating the process was determined by general practitioners without expertise in prostate cancer who were "clearly involved in political, inter-governmental discussions."
"In view of the flawed process, we believe that the Scottish government simply intended to use compassionate release as political cover for returning al Megrahi to Libya -- regardless of whether his physical condition met the requirements," the report said.
Menendez, a Democrat, said he undertook the investigation after British and Scottish officials refused to testify at a Senate Committee on Foreign Relations hearing he was scheduled to chair in July.
"Instead of living three months, he's lived 13 months and counting," Menendez said in a September committee hearing on the matter.
At the hearing, Menendez questioned what he characterized as contradictory medical information from the Scottish government.
He played a videotape that showed al Megrahi walking up a flight of stairs to board a plane to Libya after his prison release, despite allegedly being near death.
A State Department official testified that commercial concerns may have influenced Scottish consideration of al Megrahi's release.
"There were political, security and economic considerations that were involved in the decision making," said Nancy McEldowney, the deputy assistant secretary of European affairs at the State Department.
The report expanded on that allegation, saying "the threat of commercial warfare was a motivating factor."
"The U.K. knew that in order to maintain trade relations with Libya, it had to give in to political demands," the report said.
McEldowney said the United States never supported the decision of releasing al Megrahi.
Menendez said he asked representatives from oil giant BP to testify at the hearing regarding allegations that the British company urged the Scottish government to release al Megrahi in exchange for a multimillion-dollar oil deal with Libya. No one from BP would testify, Menendez said.
Al Megrahi was the only person convicted in the bombing. Most of the victims were Americans. The flight was heading to New York from Frankfurt, Germany, via London, England, when it blew up.
A special Scottish court in the Netherlands convicted al Megrahi in 2001. Released in August 2009 after serving eight years of his life sentence, he returned to Libya.
CNN's Laurie Ure contributed to this report.