A female US soldier told army investigators that photos of prisoners being abused in Iraq were taken "for fun", a US military court has heard.
The defence claims the abuses are part of a systemic problem
Private Lynndie England faces a number of charges, ranging from abusing prisoners at Abu Ghraib jail, to having photos of them in humiliating poses.
The court is back on Wednesday to seek to decide whether to court-martial her.
Pte England could face up to 38 years in prison. Her lawyers say she was acting under orders from superiors.
The US officer in charge of the Abu Ghraib prison at the time of the abuse, Brig Gen Janis Karpinski, supported the allegation, saying the conspiracy could have gone as high as the Pentagon or the White House.
Pte England's face became familiar after she appeared in photographs pointing at prisoners' genitals and holding a naked Iraqi detainee on a leash.
Rarely has a once obscure private done so much damage to the standing of the US, says the BBC's Rob Watson in Washington.
Abuse 'no big deal'
Two witnesses called by the prosecution gave evidence on Tuesday, the first day of the hearing.
Investigator Paul Arthur told the four-day hearing in North Carolina that Pte England and the seven other soldiers charged "were joking around".
England appeared in some of the most infamous photos (AP Photo/Courtesy of The New Yorker)
"They [the soldiers] didn't think it was that serious... They didn't think it was that big a deal," he said.
Asked what he had established on why the soldiers had abused the prisoners, he said: "Basically it was just for fun... and to vent their frustration."
He said Pte England had told him that one of her superiors, Spc Charles Graner had asked her to pose in the picture of the inmate on the leash.
The second witness, military crimes investigator Warren Worth, who also interviewed Pte England, was asked whether she was "a willing participant" in the photo.
"She didn't say she objected to it," Mr Worth said.
He also said that, with the exception of two immediate superiors charged over the issue - Spc Graner and Staff Sgt Ivan Frederick - he had found no evidence that the soldiers had been given orders from the top command.
"If we're talking about persons higher... I had no indication that anybody knew," Mr Worth told the court.
But under cross-examination Mr Worth said some soldiers mentioned that military intelligence had encouraged them to "soften" prisoners.
The Pentagon has denied sanctioning any rough treatment of prisoners, and describes the abuse as isolated incidents.
Pte England is to be charged with conspiracy to mistreat Iraqi prisoners, assaulting prisoners, committing acts prejudicial to good order, committing indecent acts, disobeying an order and creating and possessing sexually explicit photographs.
Her lawyers have promised to mount a vigorous defence, denying the charges and blaming flawed American policies in Iraq.
But the private, who is pregnant, was absent from the afternoon session of the court, apparently making an unscheduled visit to a doctor.
After the court adjourned, defence lawyer Richard Hernandez told journalists Tuesday's evidence had strengthened his belief that the abuses were part of a systemic problem.
"These tactics are being used at places my client has never been," he said.
He added that the government was doing everything it could to make her a scapegoat.
"MI [military intelligence] investigate MI," he said. "The fox is guarding the henhouse."
The US military says it is investigating another 94 possible prisoner abuse cases in Iraq and elsewhere.