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

GUANTÁNAMO BAY, Cuba — It started out as questioning about C.I.A. policy, contracts and cables. Then it shifted to a more visceral examination of what happened to the men accused of conspiring in the Sept. 11, 2001, attacks while they were held in secret prisons, with a former interrogator testifying about chains, shackles, hoods and threats to kill one prisoner’s son.

In a pretrial hearing on Tuesday, David Nevin, the lawyer for Khalid Shaikh Mohammed, the accused mastermind of the 9/11 plot, held up various pieces of evidence collected at one of the C.I.A.’s now-closed overseas detention and interrogation sites. He asked the witness, James E. Mitchell, a former C.I.A. contract psychologist who worked in the secret prisons and helped devise the torture program, what they were.

Shown a chain with a red lock and built-in blue metal device, Dr. Mitchell said it looked like something you could “cinch up like a horse collar” but declared the device “completely unfamiliar to me.”

His answers were much the same as he was confronted with questions about other accounts of how the prisoners were treated. Dr. Mitchell said he did not recognize a screeching rendition of the heavy metal song “Let the Bodies Hit the Floor,” which detainees claimed was blasted at them in isolation. He disputed the fictional portrayal in the recent film “The Report” of Mr. Mohammed being violently waterboarded.