WASHINGTON (Reuters) - CIA interrogators carried out mock executions and threatened an al Qaeda commander with a gun and an electric drill, according to an internal report that provides new details of abuses inside's the agency's secret prisons, two leading U.S. newspapers reported on Saturday.

The Central Intelligence Agency inspector general's report is due to be released on Monday, The New York Times and The Washington Post said on their websites, citing U.S. officials familiar with the document.

The tactics -- which one official described to the Post as a threatened execution -- were used on Abd al-Rahim al-Nashiri by CIA jailers who held the handgun and drill close to the prisoner to frighten him into giving up information.

Nashiri, who was captured in November 2002 and held for four years in one of the CIA's "black site" prisons, was one of three al-Qaeda chieftains later subjected to a form of simulated drowning known as waterboarding, the paper said.

The report, completed in 2004 by the inspector general, John L. Helgerson, also says that a mock execution was staged in a room next to one terrorism suspect. CIA officers fired a gun in the next room, leading the prisoner to believe that a second detainee had been killed, the Times said.

Details of the report were first published by Newsweek magazine on its website late on Friday.

A federal judge in New York has ordered a redacted version of the classified CIA report to be made public on Monday, in response to a lawsuit by the American Civil Liberties Union.

Lawyers for the Justice Department and the CIA have been scrutinizing the long-concealed agency report since June to determine how much of it can be made public.

Attorney General Eric H. Holder Jr. has been weighing the report's findings as part of a broader probe into the CIA's use of harsh interrogation methods.

Nashiri, who was implicated in the bombing of the Navy destroyer Cole in 2000, was one of two CIA detainees whose interrogation sessions were videotaped, but the tapes were destroyed by CIA officers in 2005, the Times said.

A federal prosecutor is now investigating the destruction of the CIA tapes, but the Justice Department has so far declined to open a formal investigation into the abuses in CIA prisons, the paper said.

The CIA declined to comment on the specifics of the report, the Times said.

"The CIA in no way endorsed behavior -- no matter how infrequent -- that went beyond the formal guidance. This has all been looked at; professionals in the Department of Justice decided if and when to pursue prosecution," CIA spokesman Paul Gimigliano told the newspaper.

(Writing by Anthony Boadle; Editing by Eric Walsh)