The Central Intelligence Agency’s attempt to keep secret the details of a defunct detention and interrogation program has escalated a battle between the agency and members of Congress and led to an investigation by the C.I.A.’s internal watchdog into the conduct of agency employees.
The agency’s inspector general began the inquiry partly as a response to complaints from members of Congress that C.I.A. employees were improperly monitoring the work of staff members of the Senate Intelligence Committee, according to government officials with knowledge of the investigation.
The committee has spent several years working on a voluminous report about the detention and interrogation program, and according to one official interviewed in recent days, C.I.A. officers went as far as gaining access to computer networks used by the committee to carry out its investigation.
The events have elevated the protracted battle — which began as a fight over who writes the history of the program, perhaps the most controversial aspect of the American government’s response to the Sept. 11 attacks — into a bitter standoff that in essence is a dispute over the separation of powers and congressional oversight of spy agencies.
The specifics of the inspector general’s investigation are unclear. But several officials interviewed in recent days — all of whom insisted on anonymity, citing a continuing inquiry — said it began after the C.I.A. took what Senator Mark Udall, Democrat of Colorado, on Tuesday called an “unprecedented action” against the committee.
The action, which Mr. Udall did not describe, took place after C.I.A. officials came to suspect that congressional staff members had gained unauthorized access to agency documents during the course of the Intelligence Committee’s years-long investigation into the detention and interrogation program.
It is not known what the agency’s inspector general, David B. Buckley, has found in the investigation or whether Mr. Buckley has referred any cases to the Justice Department for further investigation. Spokesmen for the agency and the Justice Department declined to comment.
Senator Dianne Feinstein, Democrat of California and chairwoman of the Intelligence Committee, gave few details about the dispute on Tuesday as she left a closed committee hearing on the crisis in Ukraine, but she did confirm that the C.I.A. had begun an internal review.
“There is an I.G. investigation,” she said.
Asked about the tension between the committee and the spy agency it oversees, Ms. Feinstein said, “Our oversight role will prevail.”
The episode is a rare moment of public rancor between the intelligence agencies and Ms. Feinstein’s committee, which has been criticized in some quarters for its muscular defense of many controversial intelligence programs — from the surveillance operations exposed by the former National Security Agency contractor Edward J. Snowden to the Obama administration’s targeted killing program using armed drones.