WASHINGTON — A behind-the-scenes battle between the CIA and Congress erupted in public Tuesday as the head of the Senate Intelligence Committee accused the agency of breaking laws and breaching constitutional principles in an alleged effort to undermine the panel’s multi-year investigation of a controversial interrogation program.

Chairman Dianne Feinstein, D-Calif., accused the CIA of secretly removing documents, searching committee-used computers and attempting to intimidate congressional investigators by requesting an FBI inquiry of their conduct — charges that CIA Director John Brennan disputed within hours of her appearance on the Senate floor.

Feinstein described the escalating conflict as a “defining moment” for Congress’ role in overseeing the nation’s intelligence agencies and cited “grave concerns” that the CIA had “violated the separation-of-powers principles embodied in the United States Constitution.”

Brennan fired back during a previously scheduled speech in Washington, saying that “when the facts come out on this, I think a lot of people who are claiming that there has been this tremendous sort of spying and monitoring and hacking will be proved wrong.”

The dueling claims exposed bitterness and distrust that have soared to new levels as the committee nears completion of a 6,000-page report that is expected to serve as a scathing historical record of the agency’s use of waterboarding and other brutal interrogation methods on terrorism suspects held at secret CIA prisons overseas after the attacks of Sept. 11, 2001.

The study, which cost $40 million, took four years to complete and entailed a review of 6.2 million pages of top-secret CIA operational cables, reports and other documents, concluded that the agency’s use of harsh interrogation techniques produced very little intelligence of any value, lawmakers who have read it told McClatchy Newspapers.

The program didn’t reveal the information that enabled the CIA to pinpoint Osama bin Laden’s suspected hideout in Pakistan at which the al-Qaida leader was killed by Navy SEALs in May 2011, they’ve said.

Moreover, the study found that the agency misled Congress, the Bush administration and the public about the usefulness of the interrogation techniques, they’ve said.

Displaying flashes of anger during her floor speech, Feinstein said her committee would soon deliver the report to the White House, and push for declassification of a document that lays bare “the horrible details of the CIA program that never, never, never should have existed.”

Maine Sens. Angus King, independent, and Susan Collins, Republican, both members of the intelligence committee, said they were concerned about Feinstein’s allegations.

“The allegations made by Chairman Feinstein that the CIA may have violated the law by accessing a computer network used by Senate Intelligence Committee staff are extremely troubling,” Collins said in a statement. “If they are true — let me make it clear — it is never acceptable for the CIA or any other federal agency to impede or undermine an investigation undertaken by a Committee pursuant to Congress’ constitutional oversight role.”

“Based upon the facts as I currently understand them, I find this situation extremely troubling,” King said in a statement. “At Mr. Brennan’s nomination hearing in February 2013, and in the privacy of my office, I made clear to him that I expect the CIA to be open and forthright with the Committee. I have concerns that the CIA has failed to meet this standard in its handling of the Committee’s Detention and Interrogation Study.”

PROBE LED TO TENSIONS ON BOTH SIDES

The latest dispute is in some ways a proxy for a deeper conflict over the committee’s report. The CIA and the committee are at odds over many of the report’s conclusions about the effectiveness of the interrogation program, but are battling primarily over tension that surfaced during the investigation.

Feinstein’s remarks provided the most detailed account of that investigation, describing an arrangement in which the CIA set up a secret facility in Northern Virginia with a set of computers where committee investigators were promised unfettered access to millions of operational cables, executive memos and other files on the interrogation program.

The disagreement between Feinstein and Brennan centers on whether agency employees or committee staff members — or both — abused their access to that shared network to gain an upper hand.

Feinstein implied that the CIA sabotaged the committee’s efforts from the outset, loading a massive amount of files on computers with no index, structure or ability to search. “It was a true document dump,” she said.

Over a period of years, investigators pored over the classified records furnished by the CIA, using a search tool that agency technical experts agreed to install. But U.S. officials said the committee gained access to a set of documents that the agency never intended to share, files that were generated at the direction of former director Leon Panetta as part of an effort to take inventory of the records being turned over to Feinstein’s panel.

The two sides have engaged in heated exchanges in recent days over the nature of those files and how they were obtained.

Referring to them as the “Panetta internal review,” Feinstein insisted that committee staff members discovered the documents during an ordinary search of the trove. She said they are particularly valuable because in tracking the flow of documents, CIA employees in some cases drew conclusions about their contents’ that match the subsequent interpretations made by committee staff members.

Jeremy Bash, Panetta’s former chief of staff, on Tuesday said that was never the director’s intent. Panetta “did not request an internal review of the interrogation program,” he said. “He asked the CIA staff to keep track of documents that were being provided. … He asked that they develop short summaries of the material, so that we would know what was being provided.”

‘PANETTA REVIEW FILES’ AT ROOT OF DISPUTE

Meanwhile, a letter that Brennan distributed to the CIA workforce on Tuesday raised questions about Feinstein’s claims and her awareness of how and when the committee obtained the so-called Panetta review files.

The letter, which Brennan sent to Feinstein on Jan. 27 and which was attached to a message he sent the workforce, recounts a meeting they had weeks earlier to discuss the matter. During that meeting, Feinstein said she didn’t know that the committee already had copies of the Panetta review. Brennan pushed her to explain why the panel had recently requested the files when they were already in its possession.

“You informed me that you were not aware that the committee staff already had access to the materials you had requested,” Brennan wrote, according to a copy obtained by The Washington Post. Brennan urged Feinstein to work with the agency to determine how the committee had obtained the documents, a request she ultimately rejected, officials said.

The CIA began to suspect that the committee had obtained those files this year after lawmakers referred to the supposed “internal review” publicly. U.S. officials said CIA security personnel then checked the logs of the computer system it had set up for the committee, and found the files had been moved to a part of the network that was off-limits to CIA.

“They did something to get those documents,” said a U.S. official briefed on the matter. A security “firewall was breached. They figured out a workaround to get it.” The official declined to elaborate.

Feinstein said the review documents were “identified using the search tool provided by the CIA,” but was careful not to say precisely how they were obtained. “We don’t know whether the documents were provided intentionally by the CIA, unintentionally by the CIA, or intentionally by a whistle-blower.”

She acknowledged, however, that committee investigators made hard copies of those files and whisked them away to its offices on Capitol Hill, in part because the committee had previously seen cases in which more than 900 pages of records disappeared from the database with no explanation.

Feinstein expressed outrage that the CIA referred the matter to the FBI. “There is no legitimate reason to allege to the Justice Department that Senate staff may have committed a crime,” Feinstein said, describing the move as “a potential effort to intimidate this staff, and I am not taking it lightly.”

She also noted that the referral was made by the CIA’s acting General Counsel, Robert Eatinger, who previously served as the top lawyer for the department that ran the CIA’s secret prisons, and who “is mentioned by name more than 1,600 times in our study.”

Feinstein, who has strongly supported other CIA programs including its drone campaign, said the agency may have violated Fourth Amendment protections against unreasonable searches, as well as laws against domestic surveillance.

Although Republicans on the committee initially voted in favor of launching the investigation, GOP members abandoned the effort after it began and so far none has voted to endorse it.

Sen. Marco Rubio, R-Fla., a member of the intelligence committee, said the dispute is “more complicated than what’s being put out there by Senator Feinstein or others … I don’t think anyone has a clean hand and I think it’s important for the full truth to come out. I think people may be surprised to learn that, in this case, there were no good guys and maybe two or three bad ones.”

Brennan said he had ordered the CIA’s inspector general to review the agency’s conduct. The inspector general, in turn, has issued a separate referral seeking a Justice Department review.

Asked if he would resign if the CIA was found to be in the wrong, Brennan said he would let the president decide his fate. “If I did something wrong, I will go to the president,” he said. “He is the one who can ask me to stay or to go.”