After Senate leaders learned about the intrusion in January and protested, the CIA made a criminal referral to the Justice Department, alleging improper behavior by Senate staffers. That referral, CIA inspector general David Buckley found, was based on inaccurate information and was not justified.
When internal investigators interviewed three CIA computer specialists, they exhibited "a lack of candor," the IG report said.
Those devastating internal conclusions prompted Brennan to abandon his defensive posture and apologize to Intelligence Committee leaders.
"The director said that wherever the investigation led, he would accept the findings and own up to them," said his spokesman, Dean Boyd, describing what has become a difficult moment for the nation's most prominent spy agency.
Brennan has convened an internal accountability board chaired by former Sen. Evan Bayh, D-Ind., to examine whether any CIA officers should be disciplined.
Furious Senate Democrats were unmoved, with some demanding further investigation and a public accounting from Brennan. By all accounts, the spying flap and the larger dispute over decade-old CIA interrogation practices have poisoned relations between key Democrats and the CIA.
Two Intelligence Committee Democrats, Sens. Mark Udall of Colorado and Martin Heinrich of New Mexico, called on Brennan to resign.
"I just don't have a lot of confidence in his leadership," Heinrich said in an interview after leaving a briefing on the IG report.
"What's needed now is a public apology from Director Brennan to staff and the committee, a full accounting of how this occurred and a commitment there will be no further attempts to undermine congressional oversight of CIA activities," said Sen. Ron Wyden, D-Ore., an Intelligence Committee member.
"The investigation confirmed what I said on the Senate floor in March — CIA personnel inappropriately searched Senate Intelligence Committee computers in violation of an agreement we had reached, and I believe in violation of the constitutional separation of powers," Sen. Dianne Feinstein, the California Democrat who chairs the committee, said in a statement.
At issue was a January search by agency officers of Senate computers for information gathered in the course of a Senate investigation into the CIA's interrogation techniques. The search involved a penetration of the Senate portion of a shared, classified computer network at a Northern Virginia facility that was being used to provide Senate aides access to millions of CIA documents.
The fruits of the Senate's yearslong inquiry — a summary of a classified report on post-9/11 detentions and interrogations that accuses the CIA of misconduct — is expected to be made public soon.
The CIA conducted the search after it began to suspect that Senate aides had obtained a draft internal review that the CIA believed the Senate was not entitled to see. The review included comments from CIA officers describing misgivings about the treatment of al-Qaida detainees.
As it turned out, the Senate staffers got the review thanks to a glitch in the CIA's firewall, several officials said.
The findings of the investigation by the CIA's inspector general were shared with the Justice Department, which declined to pursue criminal charges against the CIA employees, officials said.
The inspector general concluded "that some CIA employees acted in a manner inconsistent with the common understanding" about the shared computer network, Boyd said.
Boyd said there was no malicious intent behind the actions of CIA officers. He said they were trying to determine how privileged agency documents ended up on the Senate side of the CIA network and whether there was a security breach.
The CIA is generally forbidden from conducting operations on U.S. soil. One reason no criminal charges were filed, said a Senate aide who was not authorized to speak publicly and requested anonymity, is that the Senate computers were on a CIA network subject to agency monitoring. But the CIA violated its agreement not to scrutinize the Senate side of the network.
The summary of the inspector general's report does not say who ordered the CIA search of Senate computers or who conducted it. The Senate staff used the system to communicate about their investigation into what some call torture by CIA officers.
Part of the CIA's computer surveillance, officials on both sides said, involved creating a fake Senate account to review what documents Senate staffers could access.
On Tuesday, Brennan informed Feinstein and Sen. Saxby Chambliss of Georgia, the senior Republican on the committee, "and apologized to them for such actions by CIA officers as described in the (inspector general's) report," Boyd said.
The move was a turnabout for the CIA director. After Feinstein complained in March about the CIA's penetration of committee computers, Brennan said, "When the facts come out on this, I think a lot of people who are claiming that there has been this tremendous sort of spying and monitoring and hacking will be proved wrong." He added, "We wouldn't do that."
At the White House, spokesman Josh Earnest defended Brennan, pointing out that the CIA director "is the one who suggested that the inspector general investigate in the first place," and saying he continued to enjoy the president's confidence.