The head of the US Senate intelligence committee has publicly accused the CIA of improperly accessing computers used by congressional staff.
Dianne Feinstein said on the Senate floor that such activities "may have undermined the constitutional framework" of government oversight.
The Senate panel was investigating allegations of abuse during a CIA detention and interrogation programme, and a CIA internal watchdog has been tasked with looking into the alleged hacking.
A BBC Washington correspondent says the spat is remarkable because it is so public: it's highly unusual for a senior US senator - and one on the intelligence committee - to level these kind of accusations on the chamber floor.
Ms Feinstein is effectively questioning the impartiality and independence of the CIA, the correspondent says.
"I am not taking it lightly," Ms Feinstein said of the matter on Tuesday, adding that the CIA may have violated federal laws in its alleged conduct.
CIA director John Brennan has rejected the allegation.
"Nothing could be further from the truth," Mr Brennan says. "The matter is being dealt with in an appropriate way, being looked at by the right authorities, and the facts will come out."