Director of the Central Intelligence Agency Mr. John Brennan issued an extraordinary apology to leaders of the US Senate intelligence committee on Thursday, conceding that the agency employees spied on committee staff and reversing months of furious and public denials.

He acknowledged that an internal investigation had found agency security personnel transgressed a firewall set up on a CIA network, which allowed Senate committee investigators to review agency documents for their landmark inquiry into CIA torture.

The admission brings Brennan’s already rocky tenure at the head of the CIA under renewed question. One senator on the panel said he had lost confidence in the director, although the White House indicated its support for a man who has been one of Barack Obama’s most trusted security aides.

CIA spokesman Dean Boyd acknowledged that agency staff had improperly monitored the computers of committee staff members, who were using a network the agency had set up, called RDINet. “Some CIA employees acted in a manner inconsistent with the common understanding reached between [the committee] and the CIA in 2009 regarding access to the RDINet,” he said.