This article was produced in partnership with the Pulitzer Center on Crisis Reporting.

GUANTÁNAMO BAY, Cuba — It was May 2003, a year into the C.I.A. program that had led to three detainees being waterboarded hundreds of times in search of information about possible terror plots, and the agency needed to train more interrogators. At a classified prison in Afghanistan known as the Salt Pit, trainees took turns practicing “insult slaps and attention grabs” on a naked prisoner named Ammar al-Baluchi.

It was not supposed to work that way. The two psychologists who had set up the interrogation program that used waterboarding and violence and was authorized by the Bush administration had intended for interrogators to learn by trying out the techniques on each other. After that, they were to be paired with a certified interrogator as an apprentice.

“We had them practice on themselves so they would know what it felt like,” James E. Mitchell, one of the contract psychologists, testified at Guantánamo last month during a pretrial hearing in the long-delayed trial over the Sept. 11, 2001 attacks. “We didn’t have them practice on detainees.”

C.I.A. personnel at the Salt Pit nonetheless chose to train on at least two prisoners, according to recently declassified accounts. About five trainees took turns slapping Mr. al-Baluchi, spinning him around and slamming his head into a wooden or cement wall, so a supervisor could observe and “then certify class member as interrogators,” said a report on the training session read aloud during testimony in the war court.