WASHINGTON — A festering conflict between the CIA and its congressional overseers broke into the open Tuesday when Senator Dianne Feinstein, Democrat of California, chairwoman of the intelligence committee, and one of the CIA’s staunchest defenders, delivered an extraordinary denunciation of the agency, accusing it of withholding information about its treatment of prisoners and trying to intimidate committee staff members investigating the detention program.
Describing what she called a “defining moment” for the oversight of US spy agencies, Feinstein said the CIA had removed documents from computers used by Senate Intelligence Committee staff members working on a report about the agency’s detention program, searched the computers after the committee completed its report, and referred a criminal case to the Justice Department in an attempt to thwart their investigation.
The 6,300-page report has been at the center of a bitter dispute between the committee and the agency, which says it contains many inaccuracies and wants them to be corrected before it is released.
Feinstein’s disclosures came a week after it was first reported that the CIA last year had monitored computers used by her staff in an effort to learn how the committee may have gained access to the agency’s internal review of the detention and interrogation program.
Feinstein said the internal review bolstered the conclusions of the committee’s still-classified report on the program, which President Obama ended in January 2009.
For an intelligence community already buffeted by controversies over electronic surveillance and armed drone strikes, the rupture with Feinstein, one of its closest congressional allies, could have broad ramifications for the entire US intelligence apparatus.
“Feinstein has always pushed the agency in private and defended it in public,” said Amy B. Zegart, who studies intelligence issues at Stanford University. “Now she is skewering the CIA in public. This is a whole new world for the CIA.”
Feinstein, who had refused to comment on the dispute, took the Senate floor on Tuesday morning to say the agency’s actions had breached constitutional provisions for the separation of powers and “were a potential effort to intimidate.”
“How this will be resolved will show whether the Intelligence Committee can be effective in monitoring and investigating our nation’s intelligence activities, or whether our work can be thwarted by those we oversee,” she said.
Hours later, John O. Brennan, the CIA director, forcefully denied Feinstein’s assertions that the agency had carried out a broad effort to spy on the committee’s work.
“Nothing could be further from the truth,” Brennan said in response to questions during an appearance at the Council on Foreign Relations.
In her 45-minute speech Feinstein gave the fullest public account of the years of backroom jousting between her committee and the CIA over the detention program, and its use of secret prisons and brutal interrogation techniques.
The dispute came to a head in mid-January, when Brennan told members of the committee that the agency had searched computers used by committee investigators at a CIA facility in Northern Virginia, where the committee was examining documents the CIA had made available for its report.
Feinstein said Tuesday that during the meeting, Brennan told her that the CIA had searched a “walled-off committee network drive containing the committee’s own internal work product and communications” and that he was going to “order further forensic evidence of the committee network to learn more about activities of the committee’s oversight staff.”
The CIA had carried out the search to determine whether investigators may have gained unauthorized access to the internal review of the detention program. Feinstein on Tuesday disputed this allegation, saying that the document had been included — intentionally or not — as part of a dump of millions of pages the CIA had provided for the intelligence committee.
Brennan, in a January letter to Feinstein that a government official who did not want to be identified released Tuesday, said the committee had not been entitled to the internal review because it contained “sensitive, deliberative, pre-decisional CIA material”— and therefore was protected under executive privilege.
The CIA’s acting general counsel has referred to the matter to the Justice Department as a possible criminal offense, a move Feinstein called a strong-arm tactic by someone with a conflict of interest in the case.
Senator John McCain, Republican of Arizona , called the allegations “very disturbing” and suggested an independent investigative body might need to be empaneled.
Most Republicans on the Intelligence Committee, however, either refused to comment on Feinstein’s charges or said she had no right to air such grievances in public.
But such conflicts with the executive branch tend to unite congressional Republicans and Democrats like little else.
“The Senate is bigger than any one senator,” Senator Patrick J. Leahy, Democrat of Vermont, said trying to rally lawmakers behind Feinstein.
“The members of the Senate must stand up in defense of this institution, the Constitution, and the values upon which this nation was founded,” he said.