(Updates with more Brennan comment, background)
By Patricia Zengerle and Doina Chiacu
March 11 (Reuters) - A bitter dispute between the CIA and the U.S. Senate committee that oversees it burst into the open on Tuesday when a top senator accused the agency of spying on Congress and possibly breaking the law.
Senator Dianne Feinstein delivered a scathing critique of the CIA's handling of her intelligence committee's investigation into a Bush-era interrogation and detention program that began after the Sept. 11, 2001, attacks but was only made public in 2006.
"I have grave concerns that the CIA's search may well have violated the separation of powers principles embodied in the Constitution," Feinstein said in a highly critical speech on the Senate floor by a traditionally strong ally of U.S. intelligence agencies.
Feinstein, chairwoman of the Senate Intelligence Committee, said the CIA searched the panel's computers to find out how staff obtained an internal agency review that was more critical of the interrogation program than the official CIA report.
She said the Central Intelligence Agency's search may have also violated the Fourth Amendment, the Computer Fraud and Abuse Act and an executive order that prohibits the CIA from conducting domestic searches or surveillance.
Feinstein described an emergency meeting on Jan. 15 with Senator Saxby Chambliss of Georgia, the top Republican on the committee, where John Brennan, the agency's director, told the two committee leaders the agency had conducted a "search" of committee computers.
Brennan on Tuesday denied the allegations on computer hacking. "Nothing could be further from the truth. We wouldn't do that," he said in a speech at the Council on Foreign Relations think tank.
He also denied the agency was trying to thwart the release of the panel's report. "We are not trying at all to prevent its release," he said.
Feinstein's comments were the latest salvo in a long-running dispute between the intelligence committee and CIA over the agency's detention and interrogation of terrorism suspects, a program that was phased out when inmates were transferred to Guantanamo Bay Naval Base in Cuba.
The Senate Intelligence Committee's own 6,300-page report criticized some of the harsh interrogation measures used by the CIA, and Feinstein has been pushing to make its findings public.
'EFFORT TO INTIMIDATE'
Feinstein said the internal CIA review mirrored some of the same concerns outlined in her staff's report, unlike the official CIA assessment of the program.
However, as the panel moved close to declassifying some of the information - a move she said was backed by the White House - the CIA acting general counsel went to the Justice Department to complain about committee staff.
"I view the acting general counsel's referral as a potential effort to intimidate this staff - and I am not taking it lightly," Feinstein said.
Brennan, who took the helm of the CIA a year ago, said the agency was continuing to work with the committee to put out as accurate a historical record as possible.
"The CIA has tried to work as collaboratively as possible with the committee on the report. We will continue to do so," he said. "The CIA agrees with many of the findings of the report and disagrees with some others."
The dispute over the report - and revelations by former contractor Edward Snowden about sweeping electronic surveillance by the National Security Agency - have sparked debate over whether congressional oversight of U.S. spy agencies is effective enough.
Brennan delivered a vigorous defense of the CIA's commitment to working with Congress.
"My CIA colleagues and I believe strongly in congressional oversight," he said. "We are a far better organization because of congressional oversight."
But lawmakers expressed skepticism.
U.S. Representative Steny Hoyer, the second-ranking House Democrat in the House of Representatives, said Feinstein's charges should be investigated.
"It's a serious allegation, I think, and Senator Feinstein is a serious legislator, so I don't think she made it lightly."
Feinstein bristled at suggestions her staff had obtained information improperly, and said the CIA itself provided her committee with more than 6.2 million documents.
"The committee clearly did not hack into CIA computers to obtain these documents, as has been suggested in the press," the California Democrat said.
Brennan also said the agency was eager to relegate the rendition, detention and interrogation program to history.
"The CIA has more than enough current challenges on its plate, which is why, far more than any other institution of government, the CIA wants to put the rendition, detention and interrogation chapter of its history behind it," Brennan said. (Additional reporting by Susan Cornwell; Editing by Andrew Hay)