Senate Majority Leader Harry Reid (D-Nev.) wrote to the attorney general: “The CIA’s decision to access the resources and work product of the legislative branch without permission is absolutely indefensible, regardless of the context.” (J. Scott Applewhite/AP)

The tussle between the Senate and the CIA escalated Thursday when Senate Majority Leader Harry M. Reid (D-Nev.) asked the Senate’s top law enforcement official to review computers used by Senate Intelligence Committee staffers to probe the spy agency’s controversial interrogation program.

Reid has asked the Senate’s sergeant at arms, Terrance W. Gainer, who has oversight of the U.S. Capitol Police, to lead a review of an incident that prompted senators and the CIA to accuse each other of illegal activity and led both sides to ask the Justice Department to investigate the matter.

Reid’s request for a Senate investigation was revealed in letters sent late Wednesday to CIA Director John O. Brennan and Attorney General Eric H. Holder Jr and comes as aides say Reid has grown convinced that the CIA overstepped its authority by attempting to interfere in the intelligence committee’s investigation into the CIA’s controversial interrogation program. The results of the years-long investigation may be released in the coming weeks.

Sen. Dianne Feinstein (D-Calif.), who leads the intelligence panel, first publicly accused the CIA of interfering in her investigation during a dramatic Senate floor speech last week. Aides said that Reid had been directly conveying his own concerns about the situation to Brennan during their regular intelligence briefings.

Reid used his letter to Brennan to reiterate his concerns and ask that the CIA grant Gainer and his staff proper security clearances and access to computers used by committee staffers to access millions of documents describing the interrogation program. As part of the investigation, the CIA agreed to set up a bank of computers at a secret facility in Northern Virginia for investigators to review operational cables, executive memos and other files on the interrogation program.

Senate Intelligence Chair Dianne Feinstein (D-Calif.) accused the CIA of breaking the law by searching her committee's computers. The Post's Karen Tumulty, Scott Wilson, Terence Samuel and Adam Goldman explain the impact in Washington. (Julie Percha and Theresa Poulson/The Washington Post)

Reid in his letter also rejected as “patently absurd” suggestions by Brennan that committee staffers improperly accessed classified information about the agency and the interrogation program. “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,” he said.

As part of the dispute, CIA officials have referred the matter to the Justice Department.

Holder told reporters Wednesday that the Justice Department is reviewing the CIA’s referral of the case, but he would not say whether the department has decided to investigate the matter.

In his letter to Holder, Reid asked that the department “carefully examine” the accusations leveled by the Senate Intelligence Committee and the CIA against each other.

“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 to Holder. “This action has serious separation of powers implications. It is immaterial whether this action was taken in response to concerns about the Committee’s possession of a disputed document; this stands as a categorically different and more serious breach.”

Feinstein has said that she expects her committee will vote to declassify its 6,000-page report on the interrogation program by the end of the month. The investigation has been underway since 2009 and is believed to have cost taxpayers at least $50 million.