It was outrageous enough when two successive presidents papered over the Central Intelligence Agency’s history of illegal detention, rendition, torture and fruitless harsh interrogation of terrorism suspects. Now the leader of the Senate intelligence committee, Dianne Feinstein, has provided stark and convincing evidence that the C.I.A. may have committed crimes to prevent the exposure of interrogations that she said were “far different and far more harsh” than anything the agency had described to Congress.

Ms. Feinstein delivered an extraordinary speech on the Senate floor on Tuesday in which she said the C.I.A. improperly searched the computers used by committee staff members who were investigating the interrogation program as recently as January.

Beyond the power of her office and long experience, Ms. Feinstein’s accusations carry an additional weight and credibility because she has been a reliable supporter of the intelligence agencies and their expanded powers since the attacks of Sept. 11, 2001 (sometimes too reliable).

On Tuesday, the C.I.A. director, John Brennan, denied hacking into the committee’s computers. But Ms. Feinstein said that in January, Mr. Brennan acknowledged that the agency had conducted a “search” of the computers. She said the C.I.A.’s inspector general had referred the matter to the Justice Department for possible criminal prosecution. “Besides the constitutional implications,” of separation of powers, she said, “the C.I.A.’s search may also have violated the Fourth Amendment, the Computer Fraud and Abuse Act, as well as Executive Order 12333, which prohibits the C.I.A. from conducting domestic searches or surveillance.”