Chief Warrant Officer Lewis Welshofer Jr is accused of killing Major-General Abed Hamed Mowhoush in 2003.
Prosecutors say Mowhoush was placed headfirst in a sleeping bag and bound with an electrical cord for interrogation. They say he asphyxiated while Welshofer sat on his chest and occasionally used his hands to cover Mowhoush's mouth.
David Hodgkinson, a state department lawyer who advises the defence department and other nations on the Geneva Conventions, said he believes the techniques used on Mowhoush are violations.
He said: "No matter what, in Iraq there's a baseline of humanity, humane treatment."
He said the Geneva rules protect against physical and mental coercion or possibly lethal activity.
Frank Spinner, defence lawyer, said Hodgkinson did not know the full story of Mowhoush's death and that his testimony was based only on prosecution evidence.
The court-martial entered its fourth day on Thursday, with lawyers painting conflicting pictures of Welshofer: A ruthless man flouting army interrogation rules, and an interrogator pressured to help defeat a growing insurgency.
Welshofer's lawyers contend he did nothing illegal and was using a technique approved by his commander. They say Mowhoush died from an irregular heart rhythm caused by heart disease and stress.
"No matter what, in Iraq there's a baseline of humanity, humane treatment"
US state department lawyer
A witness, who testified from behind a screen to cloak his identity, testified on Wednesday that Welshofer said interrogation rules were being flouted every day in Iraq.
The witness said he spoke to Welshofer on 25 November, 2003, the day before Mowhoush's death. The witness said he asked Welshofer if he was aware of a memorandum from Welshofer's commanding general that required authorisation for the use of certain interrogation techniques.
"He said he was aware of them, but said he was pretty sure they were breaking those rules every day," the witness said.
The memo, dated 12 October 2003, did not mention stress techniques such as the sleeping bag position but said anything not in the memo required approval, according to the witness.
Welshofer's commander has said she would not have approved the sleeping bag technique to be used the way it was against Mowhoush.
Under defence questioning, the secret witness conceded he did not witness any wrongdoing.
"What Chief Welshofer told you could have been just a flippant remark, right?" Spinner asked.
"Yes," the witness replied.
"So you have no personal knowledge that any rules were being violated," Spinner said.
"No," said the man, adding that while he was alarmed by Welshofer's comment he did not report it to commanding officers.
Open to public
The dramatic testimony was opened to the public at the request of media organisations, though the judge did allow another witness to testify behind closed doors on Wednesday.
Two witnesses also testified in secret on Tuesday.
Gerold Pratt, another witness, said Mowhoush was beaten by civilian interrogators two nights before his death. Pratt, who was in charge of logistics at the detention camp, did not identify the civilians.
He said Mowhoush appeared to be injured following the beating and could not put his shoes on because his feet were so swollen. Prosecutors said the beating was directed by Welshofer.