LONDON — Ten days before the first anniversary of the release of the Lockerbie bomber, the Scottish government on Tuesday came under fresh pressure to justify its decision to release the bomber, Abdel Basset Ali al-Megrahi, on medical grounds.

The United States Senate Foreign Relations Committee and the Labor Party opposition in the Scottish assembly demanded that the government publish full details of the medical advice that led Justice Secretary Kenny MacAskill to rule that Mr. Megrahi should be set free on compassionate grounds because he was suffering from advanced prostate cancer and had less than three months to live. Nearly a year later, Mr. Megrahi, a former Libyan intelligence agent who was serving a life sentence for the December 1988 bombing of Pan Am Flight 103 in which 270 people died, remains alive in Tripoli, the Libyan capital, where he was greeted as a hero on his return.

Mr. MacAskill has said that he relied on the advice of Dr. Andrew Fraser, medical director of the Scottish prison services, who was said by the justice secretary to have relied on the judgments of cancer specialists. But the specialists’ recommendations have not been published, and recent reports have revealed that Mr. Megrahi had not yet begun a recommended course of chemotherapy when he was flown home to Libya. British specialists in prostate cancer have said that a man in his condition might live as long as 10 years.

The release prompted a surge of anger among relatives of the 189 Americans who died in the crash. The Senate Foreign Relations Committee said on Tuesday that it had sent a letter to Scotland’s first minister, Alex Salmond, asking the government to release the full medical information that led to Mr. Megrahi’s release, including names, medical training and specializations of the doctors who examined him.

“We understand that an extensive medical record was used as the basis of the decision to release Mr. Megrahi, but only one three-page medical document with redactions has been released by the Scottish government,” the letter said.

A committee hearing scheduled for last month was to have examined the role played in the release by the British government and BP, the oil giant, which was seeking to close a $900-million oil exploration deal with Libya. The hearing was postponed, however, when the leading Scottish officials involved, as well as officials in the former British government of Prime Minister Gordon Brown, refused to travel to Washington to testify.

But Britain’s new prime minister, David Cameron, who has described the release as “completely and utterly wrong,” said after talks with President Obama at the White House that he would see that any relevant British government documents in the case were made available to Senate investigators.

In Scotland, American pressure on the issue has stirred fresh controversy, with Mr. Salmond saying the BP oil deal played no part in the release. The leader of Scotland’s Roman Catholics, Cardinal Keith O’Brien, joined the debate over the weekend with remarks in which he contrasted Scotland’s “culture of compassion” with what he called a “culture of vengeance” in parts of the United States. “I would rather live in a country where justice is tempered by mercy than exist in one where vengeance and retribution are the norm,” Cardinal Murphy wrote in the newspaper Scotland on Sunday.