But soon after Alberto R. Gonzales’s arrival as attorney general in February 2005, the Justice Department issued another opinion, this one in secret. It was a very different document, according to officials briefed on it, an expansive endorsement of the harshest interrogation techniques ever used by the Central Intelligence Agency.

The new opinion, the officials said, for the first time provided explicit authorization to barrage terror suspects with a combination of painful physical and psychological tactics, including head-slapping, simulated drowning and frigid temperatures.