Adding heat on the CIA, the Senate will investigate a computer network that contained a still-secret review of US terror interrogations that led to dueling criminal referrals to the Justice Department and a dramatic collapse in relations between the nation's spy agencies and the lawmakers entrusted with their oversight.

In letters to the heads of the CIA and Justice Department, Senate Majority Leader Harry Reid said the CIA's decision to search the Senate intelligence committee's network and computers without approval was "absolutely indefensible" and carried serious implications for the separation of powers between the executive and legislative branches.

Reid said he had instructed his Senate's chief cop to examine how Senate staffers obtained an internal CIA review, which the agency accused them of improperly copying, although Reid described the CIA's alleged monitoring of Senate computers as more serious.

Meanwhile, legislative aides said the Senate intelligence committee will push soon for declassification of parts or all of its 6,000-page report on the agency's "war on terror" interrogation tactics at secret sites, the starting point of the entire dispute.

The parameters of the sergeant-at-arms's investigation are unclear and it's unknown what cooperation he'll receive from the CIA, which has been locked in a bitter rift with the intelligence committee's chairman, Sen. Dianne Feinstein (D) of California. The agency accuses committee staffers of illegally accessing certain documents; Feinstein and other senators say the CIA broke the law by monitoring its computer use and deleting files.

"To my knowledge, the CIA has produced no evidence to support its claims that Senate committee staff who have no technical training somehow hacked into the CIA's highly secure classified networks, an allegation that appears on its face to be patently absurd," Reid said in a letter, dated Wednesday, to CIA Director John Brennan. A previous review, he said, appears to corroborate committee findings and contradict CIA claims.

CIA spokesman Dean Boyd said the agency was committed to resolving its differences with senators. The CIA, he said, "believes in the necessity of effective, strong and bipartisan congressional oversight."

The clash between Congress and the CIA is arcane in its particulars but potentially broad in scope, with the sides battling over who will write the official history of one of the darkest eras in American spying — the waterboarding and brutal interrogations of Al Qaeda prisoners in undeclared "black site" prisons overseas. Feinstein's claim that the CIA has undermined separation of powers makes it a constitutional fight, too.

The disagreement had been kept under wraps until this month and broke out fully into the open after the California Democrat took the Senate floor to outline her case last week. It was an extraordinary intervention for a senator who has been among the staunchest defenders of US intelligence agencies since former National Security Agency systems analyst Edward Snowden's public revelations of massive government collection of telephone and other data.

The CIA has thus far dismissed Feinstein's allegations, and the Justice Department has said only it is reviewing the competing claims of wrongdoing. The White House has refused thus far to weigh in — even if the origins of the dispute focus on the effectiveness of interrogation methods authorized and conducted by the Bush administration in the aftermath of the Sept. 11, 2001, terror attacks.

Intelligence committee members hope President Barack Obama will help make public the 6,000-page report they are close to completing. Last week, the president said he would declassify the "findings so that the American people can understand what happened in the past, and that can help guide us as we move forward."

Congressional aides said the committee planned a vote next week calling for declassification, though it was unclear if they'd demand that action begin on the full report or a shorter, 400-page summary. They want Obama to ensure the CIA conducts the line-by-line declassification in good faith and to prevent any officials who were part of its interrogation unit from participating in the process.

The aides weren't authorized to speak publicly about the plans and demanded anonymity.

"Getting to the truth about the CIA's brutal and ineffective interrogation and detention program goes to the core of the effectiveness and integrity of the CIA as an institution," Sen. Mark Udall (D) of Colorado wrote in a letter to Obama on Thursday. He urged declassification as soon as possible.

Reid said the sergeant-at-arms would conduct a forensic investigation to determine how the previous review, known as the "Panetta review" because it was ordered by then-CIA Director Leon Panetta, entered the computer network set up for the committee at a northern Virginia site.

Committee staff reportedly took copies of those documents to the Capitol for safekeeping. Reid asked the CIA to grant the appropriate security clearances to the sergeant-at-arms, who oversees security at the Capitol and Senate. Reid said the sergeant-at-arms, Terrance W. Gainer, would step down in the spring after seven years and be replaced by his deputy, Drew Willison.

In his letter to Attorney General Eric Holder, Reid challenged the credibility of Brennan's claims. He echoed Feinstein's conflict-of-interest concerns about the CIA lawyer Robert Eatinger, who was acting general counsel when he filed the criminal referral against Senate staffers. That was after Eatinger was named 1,600 times in the committee's study of the interrogation program.

Troubled by the CIA's actions, Reid wrote to Holder, "Left unchallenged, they call into question Congress' ability to carry out its core constitutional duties and risk the possibility of an unaccountable intelligence community run amok."

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Associated Press writer Stephen Braun contributed to this report.

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