The leader of the U.S. Senate strongly backed the head of the Senate Intelligence Committee on Thursday in a dispute with the Central Intelligence Agency, ordering an investigation into what he called an "indefensible" breach of the panel's computers by the CIA.

Senate Majority Leader Harry Reid said accusations that the agency spied on the panel's computers as it investigated the use of harsh interrogation techniques during the Bush administration raised concerns about possible violations of the Constitution.

"The CIA has not only interfered with the lawful congressional oversight of its activities, but has also seemingly attempted to intimidate its overseers by subjecting them to criminal investigation," Reid said in a letter to Attorney General Eric Holder obtained by Reuters.

"These developments strike at the heart of the constitutional separation of powers between the legislative and executive branches," Reid, a Nevada Democrat, added.

Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, accused the agency last week of searching computers used by committee staffers compiling a report on the CIA's use of harsh interrogation methods such as simulated drowning, or waterboarding, in the wake of the September 11, 2001, attacks on the United States.

Questioning whether the agency broke the law, the veteran California Democrat also denied wrongdoing by members of her staff in a strongly worded speech on the Senate floor that brought to light the simmering dispute between Democratic committee members and President Barack Obama's CIA chief.

It was a striking departure for Feinstein, one of the U.S. lawmakers most supportive of the national security community, notably since former contractor Edward Snowden began releasing secret information about U.S. surveillance programs.

Republican committee members have mostly stayed out of the fray, as has Obama. The president said it would be inappropriate to weigh in on an ongoing investigation.

The CIA accused committee staff members of illegally obtaining information, and sent a "crimes report" to the Justice Department about committee staff members' activities in obtaining classified information.

'PATENTLY ABSURD'

The Intelligence Committee is responsible for oversight of the CIA. Its 6,000-page draft report on the interrogation program was finished over a year ago but is still classified. Sources say it strongly condemns the now-abandoned harsh interrogation techniques and concludes they did not produce significant counter-terrorism breakthroughs.

Reid said he would have the Sergeant at Arms, the Senate's chief security officer, begin a forensic examination of computers assigned for exclusive use by Senate Intelligence staff.

"The CIA's decision to access the resources and work product of the legislative branch without permission is absolutely indefensible, regardless of the context," Reid wrote.

He said the agency's actions cause him "great concern" as leader of the U.S. Senate.

Reid also wrote to John Brennan, the director of the CIA, asking for cooperation with the investigation of what Reid said would mark the third time since 2010 in which the agency acknowledged intruding into Intelligence Committee computer networks.

Both letters were dated March 19.

"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 wrote to Brennan.

The CIA responded that Brennan is committed to resolving every outstanding issue related to the Intelligence Committee's report and strengthening its relationship with Congress.

"We are a far better organization because of congressional oversight, and we will do whatever we can to be responsive to the elected representatives of the American people," CIA spokesman Dean Boyd said.

The Justice Department had no immediate response to Reid. "We are reviewing the letter," department spokesman Brian Fallon said in an email.

Attorney General Holder told a news conference on Wednesday that Justice was reviewing the referrals from the CIA on the Intelligence Committee's interrogation review. He said receiving a referral does not mean the department would investigate.