The CIA has conceded it improperly monitored computers used by the US Senate Intelligence Committee in an investigation of CIA interrogation tactics and secret prisons for terrorism suspects after the September 11, 2001 attacks.
Central Intelligence Agency spokesman Dean Boyd said in a statement that the agency's inspector general had determined that "some CIA employees acted in a manner inconsistent" with an understanding between the agency and the Senate panel.
Boyd said CIA Director John Brennan had informed Senator Dianne Feinstein, the Senate committee's chair, and its senior Republican, Senator Saxby Chambliss, of the finding and apologised.
The Senate committee has been investigating excesses allegedly committed by CIA officers who used harsh interrogation methods, including waterboarding or simulated drowning, and established a network of secret prisons abroad.
Human rights activists and critics of the CIA's methods, including some US politicians, have described the CIA's interrogation methods as torture.
According to an unclassified summary of the inspector general's report obtained by Reuters, he found that five agency employees, two lawyers and three information technology staffers, had "improperly accessed" a data network Senate investigators were using to pursue their inquiry.
The special data network was set up by the CIA in one of its offices for the purposes of the congressional investigation.
The summary says that the CIA's Office of Security also looked at how Senate investigators had accessed the data network and conducted a "keyword search of all and review of some" of the congressional investigators' emails sent through the network.
As tension built between the CIA and the committee earlier this year, the agency asked the Justice Department to open a criminal investigation into whether committee staffers had used the data network to access privileged CIA information.
However, the inspector general's summary said it turned out that the "factual basis" for the criminal referral which the agency sent to the Justice Department "was not supported" because the lawyer who made the referral "had been provided inaccurate information."
White House press secretary Josh Earnest said that Brennan, who worked as counter-terrorism advisor to President Barack Obama before becoming CIA chief, had "done what is necessary to get to the bottom what had happened".
The CIA said Brennan had ordered a further inquiry, headed by former Senator Evan Bayh, to see if disciplinary actions or institutional reforms were needed.
Senator Mark Udall, a Democrat on the intelligence committee, called upon Brennan to resign, saying the activity demonstrated "a tremendous failure of leadership".
The White House is expected to deliver a declassified summary of the committee's report, and the CIA and Republican responses, to Congress by the end of this week.
Officials familiar with the committee's report say it concludes that the use of coercive interrogations did not produce any significant counter-terrorism breakthrough in the years after the 2001 al Qaeda attacks.
According to the officials, the report says CIA officials misstated or exaggerated the results of the programme to other agencies and to Congress.