David Passaro is charged with beating Abdul Wali during two days of questioning in June 2003 at the base in Asadabad, Afghanistan. He is the first American civilian charged with mistreating a detainee during the wars in Iraq and Afghanistan.
The CIA officer, who said he was in charge of the agency's operations at Asadabad, testified in disguise on Wednesday and used the pseudonym Steve Jones. He said Passaro readily volunteered to take over questioning Wali after an initial interrogation produced no information on rocket attacks.
He said he allowed Passaro to continue his questioning until learning that Wali was "not doing well".
"He was down in his cell and he was groaning," the officer testified.
Wali died later that day. According to Jones, Passaro said he had knocked the prisoner down once during his two days of questioning and that Wali had been caught with a makeshift key trying to open his shackles.
"He mentioned to me that he had to defend himself," Jones said. "He said Wali had attempted to lunge at him and he had to knock him down."
Earlier on Wednesday, a retired Army Special Forces soldier testified that Passaro had become enraged when Wali was not able to answer questions about the rocket attacks, but said he never saw Passaro strike the prisoner.
"It became clear he [Wali] was not going to be a font of information," said retired Chief Warrant Officer Brian Halstead.
"Dave starts getting mad, real mad. Dave starts hollering. Dave is screaming at this guy. ... Red in the face, spit flying, finger-poking. He was going off."
Halstead, who said he was in charge of planning operations in Afghanistan's Kunar province at the time, said he was in the room with Passaro and Wali as Hyder Akbar, the son of provincial governor Fazel Akbar, tried to translate.
Halstead's account of Wali's interrogation matched that of the younger Akbar, who testified on Tuesday that Passaro was "full of rage" during questioning.
Hyder Akbar said he accompanied Wali to the American base after the governor persuaded Wali to try to clear his name. But four days later, the son received a call from Passaro asking him to return and pick up his body.
Akbar, now a student at Yale University, said Wali insisted he was innocent of the rocket attacks.
During cross-examination on Wednesday, the younger Akbar told Passaro's public defender he could not vouch for Wali's whereabouts during the rocket attacks.
Defence lawyer Joe Gilbert also asked Akbar about his book, Come Back to Afghanistan, which includes a chapter about Wali. Akbar said the book had nothing to do with his reasons for testifying.
Prosecutors have charged Passaro with two counts of assault with a dangerous weapon and two counts of assault resulting in serious injury. If convicted, the 40-year-old faces up to 40 years in prison.