The details of the treatment of dozens of detainees at the hands of American intelligence contractors are by now widely known, yet it is still chilling to watch Dr. Mitchell and Dr. Jessen rationalize their use of techniques that the C.I.A.’s top lawyer at the time called “sadistic and terrifying.”

“I thought he would be uncomfortable,” Dr. Mitchell said of waterboarding, in which torturers simulate the sensation of drowning by pouring water over a cloth covering a person’s face. “It sucks. I don’t know that it’s painful, but it’s distressing.” Dr. Mitchell once said detainees would rather have their legs broken. A 2002 cable described the waterboarding of Abu Zubaydah, who officials wrongly believed was a leader of Al Qaeda, and who was subjected to the procedure 83 times over a matter of days. “At the onset of involuntary stomach and leg spasms, subject was again elevated to clear his airway, which was followed by hysterical pleas. Subject was distressed to the level that he was unable to effectively communicate or adequately engage the team.”

Mr. Zubaydah did give interrogators key information about the Sept. 11 plot — not as a result of the waterboarding, but in response to traditional interrogation methods. Yet, thinking he might have more, torturers forged ahead with Mr. Zubaydah and with others, confident that physical abuse would lead to actionable intelligence. Some detainees were handcuffed to a bar on the wall so they could not rest or lie down for days at a time. During his own deposition, Jose Rodriguez, a top C.I.A. official who destroyed videotapes of the interrogations because of what he called their “ugly visuals,” compared the abuse to a gym workout. When Suleiman Salim, one of the plaintiffs suing the psychologists, was asked to describe his experience, he broke down in tears.