“I was making recommendations to a team who were doing the interrogation,” he said. “But there was intense pressure for results. There was a tremendous amount of pressure not to let other Americans die.”

That summer, as Justice Department lawyers and the White House finalized the legal memos justifying the interrogations, Mr. Mitchell said he gave a presentation outlining an aggressive approach. He disputes the notion that he pushed the agency down a road it did not want to go. “It was clear to me from walking the halls that they were going to use coercive interrogations,” he said. “It was clear that was the direction they were going.”

The SERE techniques, he said, were an attempt to standardize the interrogation process and bolster it with research. “I said, if you are going to use coercive techniques, then don’t let people just freelance,” Mr. Mitchell recalled. “Use something that people have a track record with.”

There was broad consensus among behavioral scientists, however, that torture did not work — subjects became so eager to stop the pain that they did not provide accurate information. And Mr. Mitchell was proposing to take techniques employed in simulations and use them for actual interrogations.

The Senate report indicates that at least some information suggesting that SERE methods were ineffective as interrogation tactics was never shared with the Justice Department. Nevertheless, the department authorized the techniques, and the C.I.A. asked Mr. Mitchell to use them.

“After a lot of soul searching, I agreed to do it,” Mr. Mitchell said. “But I knew that at that moment, my life as I knew it was over. I went through my ethical obligations, and decided for me, the least worst choice was to help save American lives. It felt like something was going to happen at any minute. I felt like you had to do something.”

Mr. Mitchell suggested that the C.I.A. also hire Mr. Jessen, a friend and former colleague. In the Air Force, Mr. Jessen had helped screen the instructors who posed as interrogators. Occasionally, he played the interrogator himself, and was once called out by colleagues for being too aggressive. Mr. Jessen did not respond to repeated interview requests.