CIA interrogators threatened a captured al-Qaida leader with a power drill and a pistol in what was described as a mock execution, according to a long-suppressed report due to be released on Monday.

Details of the report by the spy agency's inspector general have emerged in the Washington Post and Newsweek. The full findings on the CIA's interrogation programme are to be made public after a federal judge upheld an appeal from the American Civil Liberties Union for their release.

The report is understood to describe mock executions where interrogators tried to get detainees to talk by firing a gun in an adjoining room to pretend another prisoner had been killed.

According to leaked information from the report, Abd al-Rahim al-Nashiri was threatened with a drill and gun during his detention at one of the CIA's so-called black site prisons after his capture in 2002. He was subjected to the near-drowning technique known as waterboarding, as were two other al-Qaida leaders.

Nashiri, who remains in detention at Guantánamo Bay, has been accused of masterminding the 1999 bombing of the USS Cole, which killed 17 American sailors.

Sources familiar with the report told the Washington Post that Nashiri was threatened with death or grave injury during his questioning. A CIA officer showed Nashiri a gun and suggested he would be shot, and a power drill was held near Nashiri's body and repeatedly turned on and off. US law on torture prohibits a US national from threatening anyone in his custody with imminent death.

The disclosures come as the CIA faces intense scrutiny. The US attorney general, Eric Holder, has been examining the legality of the CIA's interrogation methods.

The inspector general examined CIA techniques over a period of two years – from 2002 until 2004 – to see whether justice department guidelines for so-called "enhanced interrogations" had been followed. Those guidelines were finally released by the Obama administration despite the objections of the CIA and former senior officials under George Bush.

The report is understood to be the most detailed review of the agency's interrogation programme and is believed to be highly critical of the techniques used, suggesting that a number of them broke international laws and norms. The document has become deeply controversial within the CIA itself, not least because the agency was advised two months before Nashiri's capture in a memo from Jay Bybee, the head of the justice department's office of legal counsel, that threats of "imminent death" were legal if they did not cause permanent mental harm.

The report – originally commissioned by then CIA director George Tenet – has become a cause celebre. It was seen by justice department and congressional intelligence committee leaders shortly after it was written, but not shown to all members of the intelligence committees until September 2006.

Top Bush CIA officials, including Tenet's successors as CIA director, Porter Goss and General Michael Hayden, lobbied for the report to be kept secret, claiming its release would damage America's reputation around the world and damage CIA morale.

Its public release comes after revelations last week that the CIA hired the private military contractor Blackwater – now known as Xe Services – to assassinate al-Qaida leaders. The programme never got off the ground and was kept secret from Congress.

Previous scandals that damaged the reputation of the CIA and the US internationally during the Bush years include the disclosure of the US secret rendition programme for terrorist suspects, the existence of the black site prisons and the use of waterboarding.

Barack Obama has said that waterboarding constitutes torture and is therefore forbidden under US law.

In Europe, the Swiss senator who has led an inquiry across the continent into secret CIA-run detention centres has urged European nations to come clean about their involvement "in this shameful episode".

Dick Marty said Europe's credibility was being damaged by leaks about CIA interrogation facilities in countries such as Poland, Romania and Lithuania. Marty said that instead of having the truth trickle out gradually, all participants in the illegal program should publicly admit their involvement.

In a 2007 probe conducted on behalf of the Council of Europe, Marty accused 14 European governments of permitting the CIA to run detention centres or conduct rendition flights through their countries between 2002 and 2005.