GLASGOW—Scotland released the convicted Lockerbie bomber from prison in August 2009 on the grounds he likely had three months to live, even though there was no consensus among specialists treating his prostate cancer that his prognosis was so dire, according to publicly available documents and people familiar with the case.

Abdel Baset Al-Megrahi was released from prison on Aug. 20. 2009, because a medical report concluded that he had three months to live, qualifying him for Scottish guidelines on "compassionate release." He was convicted in 2001 to a life sentence with a minimum 27 years imprisonment.

The prognosis was made by Andrew Fraser, a doctor who administers Scotland's prison health service, and became the sole medical basis for Justice Secretary Kenny MacAskill's controversial decision to set Mr. Megrahi free.

The release of Mr. Megrahi— the only man ever convicted in the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, that killed 270 people—set off an international furor that continues today in part because Mr. Megrahi remains alive and free in Tripoli. The U.S. Senate Foreign Relations Committee is investigating the circumstances behind Mr. Megrahi's release.

In his report, Dr. Fraser portrayed his conclusion as the outcome of a detailed consultation with Mr. Megrahi's doctors and other specialists. But an examination of the report shows that one key group never explicitly assented to Dr. Fraser's conclusion: the specialists from the U.K. National Health Service who treated Mr. Megrahi's cancer.

The case has attracted scrutiny because of suspicions that U.K. political and commercial interests, or the influence of troubled oil giant BP PLC, played a role in securing Mr. Megrahi's release. BP has acknowledged pushing for a controversial prisoner-transfer accord that was struck between the U.K. and Libya, which could have been used to send Mr. Megrahi home. But Mr. Megrahi won his "compassionate release" because of his illness, not the prisoner-transfer agreement. Scotland, the U.K. and BP all say the oil giant had nothing to do with it.

While there was no question that Mr. Megrahi's cancer was terminal, there is no evidence that any of his specialists—two urologists and two oncologists—gave or signed off on a three-month prognosis, a review of the records and interviews with people familiar with the case indicate. According to people familiar with the matter, neither of the urologists offered any prognosis or was asked for one.

Dr. Fraser's report says "no specialist 'would be willing to say'" whether Mr. Megrahi had more or less than three months to live. The report cites the observations of Mr. Megrahi's primary-care physician at Greenock prison, a young doctor who reported that Mr. Megrahi's clinical condition "declined significantly" during a one-week period in late July and early August last year. That clinical decline is not described in detail.

Dr. Fraser and Mr. MacAskill declined to comment. Scotland defends its prognosis, but declines to say whether any doctor agreed with Dr. Fraser's crucial assessment. "Dr. Fraser drew on expert advice from a number of cancer specialists in coming to HIS clinical assessment that a three-month prognosis was a reasonable estimate," a spokeswoman for the Scottish government said in an email.

Meanwhile, in his application for compassionate release, Mr. Megrahi said he was planning to begin chemotherapy at his doctor's suggestion. That, prostate cancer experts say, would suggest he wasn't three months from death: For patients such as Mr. Megrahi—with advanced metastatic prostate cancer resistant to hormone treatment—the median life expectancy from the start of chemotherapy is 19.2 months, according to a recent study. Dr. Fraser didn't address the planned chemotherapy in his report.

The Scottish government has released Dr. Fraser's three-page report but not any specific medical information about Mr. Megrahi that would let the public see what information Dr. Fraser had before him. Sen. Robert Menendez (D., N.J.), who is leading a Foreign Relations Committee investigation into the release, called Thursday for the release of those documents. He said Scotland initially agreed to help by providing information about Mr. Megrahi's release but subsequently decided not to answer questions. "We would like to pursue and get all of his medical records released, because it would give us the wherewithal to come to a more accurate picture," Mr. Menendez said.

Families of Lockerbie victims have long been suspicious of the three-month prognosis, which has received little scrutiny against the backdrop of the BP allegations.

"It was a scam," said Eileen Walsh of Glen Rock, N.J., in an interview Thursday. Her father, brother and pregnant sister died on Pan Am 103. "The man may have cancer, but the condition and the extent of it were greatly exaggerated." She added: "Release his medical records and prove me wrong."