The director of the CIA, offering a rare apology, has acknowledged an internal probe's findings that CIA employees in the Executive Branch improperly spied on the Legislative Branch by searching Senate computers and reading staffers' emails earlier this year.

According to a declassified CIA inspector general's report, CIA officers improperly accessed Senate computers, read the emails of Senate staff, and exhibited a "lack of candor" when interviewed by agency investigators. The document, released Thursday by the CIA, is a summary of an internal CIA investigation -- which prompted CIA Director John Brennan to abandon his defiant posture in the matter and apologize to Senate Intelligence Committee leaders.

Brennan also has convened an accountability board that will investigate the conduct of the CIA officers and discipline them, if need be.

But the admission already has led to fierce recriminations from Senate lawmakers.

Sen. Mark Udall, D-Colo., said he has "lost confidence" in Brennan, and urged the administration to appoint an independent counsel to investigate.

By Thursday afternoon, he had called for Brennan's resignation.

“The CIA unconstitutionally spied on Congress by hacking into the Senate Intelligence Committee computers," Udall said. "This grave misconduct not only is illegal, but it violates the U.S. Constitution’s requirement of separation of powers. These offenses, along with other errors in judgment by some at the CIA, demonstrate a tremendous failure of leadership, and there must be consequences.”

Sen. Ron Wyden, D-Ore., called for a "public apology" from Brennan.

"The CIA Inspector General has confirmed what Senators have been saying all along: The CIA conducted an unauthorized search of Senate files, and attempted to have Senate staff prosecuted for doing their jobs," Wyden said. "Director Brennan's claims to the contrary were simply not true."

The forthcoming CIA inspector general report examined conflicting allegations that CIA personnel and Senate intelligence committee staff each improperly accessed documents.

The report determined agency officers searched Senate computers without permission for information gathered in the course of a Senate investigation into the CIA's interrogation techniques. Five agency employees improperly accessed Senate computers in an effort to track down certain documents, the inspector general found. Then, after Brennan ordered a halt to the review, the CIA office of security began a "limited investigation" that led to surveillance of Senate emails, the report said.

Three information technology staff "demonstrated a lack of candor about their activities" in interviews with CIA investigators, the report said.

The CIA inspector general shared his findings with the Justice Department, which has so far declined to pursue criminal charges against the CIA employees, officials said.

An agency spokesman said the report concluded "that some CIA employees acted in a manner inconsistent with the common understanding reached between" the committee and the CIA in 2009 regarding access to a shared classified computer network. The CIA penetration occurred after the aides got ahold of documents that the CIA claimed were internal, but which showed that some CIA officials shared misgivings about the treatment of Al Qaeda detainees.

The development was the subject of wildly different characterizations by sources on either side of the dispute.

Senate aides familiar with the matter say the CIA used classified "hacking tools" and created a fake user account in an effort to retrieve documents the CIA believed the Senate staffers had improperly accessed.

A U.S. official familiar with the inspector general report disputed that hacking tools were used, and said that there was no malicious intent behind the CIA actions, but simply an effort to account for documents believed to have been improperly accessed.

The Associated Press contributed to this report.