President Barack Obama's nominee to head the Central Intelligence Agency, John Brennan, is expected to face tough questioning about leaks of sensitive information and U.S. spy activities from waterboarding to the use of drones when he appears at a Senate confirmation hearing on Thursday.

In a written submission posted on the Senate Intelligence Committee's website on Wednesday, Brennan acknowledged for the first time that he had given voluntary interviews in connection with investigations into leaks that are being conducted by federal prosecutors in Baltimore and Washington.

Brennan said the investigations related to cyber warfare against Iran and a foiled bomb plot tied to al Qaeda's Yemen-based affiliate. Brennan said his lawyer had been told by prosecutors that he was "only a witness" in both investigations.