WASHINGTON -- Top Republican senators said on Monday they were troubled by Attorney General Eric Holder's decision to begin a new criminal probe of past interrogation tactics used by the CIA during President George W. Bush's war on terror, and expressed concern it could hamper U.S. intelligence efforts.

A newly declassified version of a CIA report revealed Monday that CIA interrogators once allegedly threatened to kill the Sept. 11 attack mastermind's children and suggested another would be forced to watch his mother sexually assaulted.

The fresh crop of damaging revelations only intensified the long-running political fight about the secret interrogation program -- whether it protected the United States then, and whether spilling its secrets now will weaken the nation's future security.

Former Vice President Dick Cheney refuted Holder's decision, saying it "serves as a reminder, if any were needed, of why so many Americans have doubts about this administration's ability to be responsible for our nation's security."

Cheney told The Weekly Standard, a conservative journal, "The activities of the CIA in carrying out the policies of the Bush Administration were directly responsible for defeating all efforts by Al Qaeda to launch further mass casualty attacks against the United States. The people involved deserve our gratitude. They do not deserve to be the targets of political investigations or prosecutions."

Holder said Monday he had chosen a veteran prosecutor, John Durham, to open a preliminary investigation to determine whether any CIA officers or contractors should face criminal charges for crossing the line on rough but permissible tactics. Durham already is investigating the destruction of CIA interrogation videos.

At the same time, President Barack Obama ordered changes in future interrogations, bringing in other agencies besides the CIA under the direction of the FBI and to be supervised by his own national security adviser. The administration pledged that questioning would be controlled by the Army Field Manual, with strict rules, and said the White House would keep its hands off the professional investigators doing the work.

Despite the announcement of the criminal probe, White House aides declared anew that Obama "wants to look forward, not back" at Bush-era tactics.

White House officials said they plan to continue the controversial practice of rendition of suspects to foreign countries, though they said that in future cases there would be greater safeguards to ensure such suspects are not tortured.

Monday's five-year-old report by the CIA's inspector general, newly declassified and released under a federal court's orders, described severe tactics used by interrogators on terror suspects after the Sept. 11, 2001, attacks. Seeking information about possible further attacks, interrogators threatened one detainee with a gun and a power drill, choked another and tried to frighten still another with a mock execution of another prisoner.

And other once-secret documents released late Monday show that parts of the CIA's tough treatment program continued even after Bush's September 2006 transfer of agency prisoners to the U.S. military base at Guantanamo Bay, Cuba.

Former CIA Director Michael Hayden, appointed by Bush in 2006, expressed dismay at the prospect of prosecutions for CIA officers. He noted that career prosecutors already had reviewed and declined to prosecute the alleged abuses.

Obama has said interrogators would not face charges if they followed legal guidelines, but the report by the CIA's inspector general said they went too far -- even beyond what was authorized under Bush era Justice Department legal memos that have since been withdrawn and discredited. The report also suggested some questioners knew they were crossing a line.

"Ten years from now we're going to be sorry we're doing this (but) it has to be done," one unidentified CIA officer was quoted as saying, predicting the questioners would someday have to appear in court to answer for such tactics.

The report concluded the CIA used "unauthorized, improvised, inhumane" practices in questioning "high-value" terror suspects.

In one instance cited in the new documents, Abd al-Nashiri, the man accused of being behind the 2000 USS Cole bombing, was hooded, handcuffed and threatened with an unloaded gun and a power drill. The unidentified interrogator also threatened al-Nashiri's mother and family, implying they would be sexually abused in front of him, according to the report.

The interrogator denied making a direct threat.

Another interrogator told Sept. 11 attack mastermind Khalid Sheikh Mohammed, "if anything else happens in the United States, 'We're going to kill your children,"' one veteran officer said in the report.

Death threats violate anti-torture laws.

Investigators credited the detention-and-interrogation program for developing intelligence that prevented multiple attacks against Americans.

"In this regard, there is no doubt that the program has been effective," investigators wrote, backing an argument by former Cheney and others that the program saved lives.

But the inspector general said it was unclear whether so-called enhanced interrogation tactics contributed to that success. Those tactics included waterboarding, a simulated drowning technique that the Obama administration says was torture. Measuring the success of such interrogation is "a more subjective process and not without some concern," the report said.

The report described at least one mock execution, which would also violate U.S. anti-torture laws. To terrify one detainee, interrogators pretended to execute the prisoner in a nearby room. A senior officer said it was a transparent ruse that yielded no benefit.

The Associated Press contributed to this report.