One of the few heroes to emerge out of the Abu Ghraib prison scandal testified Friday that he anonymously reported members of his military police unit to investigators and turned over a series of disturbing photographs because he wanted to put an end to the abuse of detainees at the facility in Iraq.

Sgt. Joseph Darby said he agonized over whether to go to authorities with disturbing digital photographs that captured his friends and colleagues in the 372nd Military Police Company beating and humiliating detainees. Darby left a letter and a compact disc with investigators in early January, nearly a month after he discovered the images. It was a move that spurred a series of military probes, scrutiny of American detention facilities, and widespread concern about U.S. policies in Iraq and around the world.

"It was a hard call to have to make the decision to put your friends in prison, ma'am," Darby said by telephone, answering questions from military prosecutors and explaining that the abuse shook him. "It violated everything I personally believed in and everything I was taught about the rules of war. It was a moral thing more than anything."

Darby broke months of public silence when he testified during the fourth day of a preliminary court hearing here for Pfc. Lynndie R. England, 21, a military police administrative clerk who has been charged with a series of abuses and indecent acts. Army Col. Denise Arn heard 25 witnesses this week and could hear more in coming days as she weighs whether to send as many as 19 criminal charges to a court-martial. If convicted, England could face as many as 38 years in prison.

The hearing has focused on the soldiers of the 372nd and their roles at the Abu Ghraib prison, a bleak place on the outskirts of Baghdad that has been described as crowded and underfunded. The prison lacked food, water, clothing and cleaning products. Some detainees opted to remain naked rather than wear burlap-like jumpsuits or female underwear -- the only clothes provided by the Coalition Provisional Authority, according to testimony.

The widely publicized sexual humiliation of detainees and abusive practices there also have opened up questions about the role of military intelligence interrogators at the facility, with defense attorneys arguing that the low-level MPs were following their orders to soften up detainees for interrogations. Although most MI officials who have testified said there was no policy that called for having MPs involved in such practices, some of the techniques used in the abuse appear similar or parallel to approved harsh interrogation practices.

England's defense attorneys said Friday that they believe the abuse is linked to high-level orders and to sexual pranks in the MP company. Soldiers testified that they often mocked gay sex acts, mooned each other, used fruit props to mimic sex and found all of it funny.

Capt. Donald Reese again testified about seeing a dead detainee who had been interrogated in a shower room and said that he believed one of MI's tactics was to keep detainees naked to humiliate them. Reese also said officials kept detainees from the International Committee of the Red Cross, with as many as 10 detainees hidden at once.

England told military investigators that she believed the abuse was being carried out at the direction of military intelligence officials, and that she was involved in a sanctioned "humiliation tactic." The abuses took place on Tier 1, in a shower room and along a stairwell, places where MI officials said authorized interrogations also took place.

Military prosecutors have presented their case by highlighting the idea that England and six other MPs were a rogue bunch that delighted in having naked, hooded detainees masturbate and in piling them in a naked human pyramid. England is shown in an iconic photograph holding a leash tied to a detainee's neck.

Darby, whom members of Congress and high-ranking Pentagon officials have praised as a conscientious soldier who did the right thing, testified that he was seeking photographs of the unit's travels through Iraq when he went to Spec. Charles A. Graner Jr. and asked him for any digital images he had. After Graner turned over two CDs, Darby found the now-infamous images of abuse.

What worried Darby was that Graner -- identified in testimony as the ringleader of the abuse -- was supposed to be assigned back to the prison, and Darby did not want the actions to continue.

"I was concerned about the abuse starting again," Darby said.

Another of the praised soldiers at the prison -- Sgt. Hydrue Joyner -- testified that conditions at Abu Ghraib were horrible and that he tried to make the best of it, often giving clothes back to inmates who were stripped by soldiers on the night shift.

Joyner said the MP unit was not trained for a correctional facility mission and that people were confused about who was in charge of Tier 1A. "No one knew what to do," Joyner said. "I was just shooting from the hip and hoping I didn't screw up."

Pfc. Lynndie R. England faces up to 38 years in prison on charges of maltreatment of Iraqi detainees at the Abu Ghraib prison in Baghdad.