In a statement, the CIA said its employees had "acted in a manner inconsistent with the common understanding reached" between the agency and lawmakers in 2009, when a Senate committee started an investigation into the CIA's harsh interrogation techniques.
Top senators were fuming over report of CIA spying over them, which was reported by the CIA Inspector General.
"This is appalling and deeply threatening to the system of checks and balances," Senate Majority Leader Harry Reid said.
"What is even more disturbing is that the unauthorized CIA actions come in the context of the Senate's effort to complete a report of the CIA's interrogation program.
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The deeply troubling CIA actions show to what lengths some in the CIA are willing to stoop in order to prevent the report's release and to avoid accountability," Reid said.
Senate Intelligence Committee Chairman, Dianne Feinstein, who was briefed Tuesday by CIA Inspector General David Buckley on the results of an IG investigation, said the CIA personnel inappropriately searched Senate Intelligence Committee computers in violation of an agreement they had reached.
Senator Patrick Leahy, Chairman, Senate Judiciary Committee, said he is appalled that the CIA searched the computers of Senate staffers who were working to shed light on "a very dark chapter" in the nation's history.
"Congressional oversight of the executive branch, without fear of interference or intimidation, is fundamental to our nation's founding principle of the separation of powers. The CIA's misconduct threatens the institution of the Senate and its role in ensuring the proper oversight of our government. The CIA Director owes the Senate and the American people an explanation of how this was allowed to occur," he said.