Aug. 6, 2011  -- Adam Winfield, the Army specialist who warned his parents that soldiers in his unit were executing innocent Afghan civilians, pled guilty Friday to reduced charges and was sentenced to three years in prison for his role in one of the murders.

Winfield, 23, of Cape Coral, Fla., had been charged with premeditated murder, which carried a sentence of life in prison without parole.

Winfield agreed to a plea deal with military prosecutors at Joint Base Lewis-McChord in Washington state, and is expected to testify against Sgt. Calvin Gibbs, who is charged with planning and executing three Afghan civilians between January and May 2010.

At Friday's hearing, Winfield told the court he has failed to stop Gibbs and another soldier, Cpl. Jeremy Morlock, from killing a detained Afghan.

"It is my duty as an American soldier ... to protect any detainee ... that is in the custody of U.S. personnel," Winfield said. "It was my job to do that, sir, and I failed to do it."

Prosecutors allege that the Ft. Lewis-based Stryker brigade set up scenarios to kill unarmed Afghans, and then planted weapons to make the killings appear justified. Winfield had been charged with premeditated murder, but said he never fired his weapon at the victim. In his plea agreement, prosecutors accepted his defense.

Winfield contended that he had not shot the Afghan victim but had also not stopped his fellow soldiers from murdering an Afghan man just outside his home in southern Afghanistan.

"Had I done my duty ... to defend that man, then it would not have happened. He wouldn't be dead," Winfield said Friday.

Winfield is one of five Lewis-McChord soldiers accused in the three killings, and the second to accept a plea deal. Morlock pled to three counts of premeditated murder in March and was sentenced to 24 years in prison. In 2010, ABC News published video of Morlock describing the "kill team"'s alleged actions. This spring photos of the men posing with corpses surfaced in the media.

Prosecutors allege that Winfield, Gibbs, Morlock, Spc. Michael S. Wagnon II, and Pfc. Andrew H. Holmes participated in one or more of the murders and staged them to make unarmed Afghans appear to be armed insurgents.

Winfield was the first to come forward about the alleged sport killings. He told his parents while deployed that members of his unit had planned and executed the killings for sport. He was charged with murder for his part in the third and final death in May of last year.

Military prosecutors are hoping Winfield's testimony in exchange for the plea agreement will help convict Gibbs, who is charged in all three Afghan deaths and believed to be the ringleader of the group.

READ: 'Repugnant' Photos Emerge of U.S. Soldiers Accused of Sport Killings

Winfield and Morlock have told Army investigators that Gibbs developed and rehearsed the murder of unarmed Afghans and later ordered his men to help him carry them out. Gibbs, Wagnon and Holmes have each pleaded not guilty to the charges.

Lawyers for Gibbs, Wagnon and Holmes have previously assailed Morlock's testimony against their clients, noting he requested investigative documents prior to testimony.

"I was wondering how can you make Morlock any less credible," Colby Vokey, the defense attorney for Wagnon, told McClatchy News. "It would be giving him copies of transcripts and having prosecutors tell him what to say."

Video: Soldier Describes 'Crazy' Sergeant's Plan

In a confession taped last year and obtained by ABC News, Morlock, of Wasilla, Alaska, admitted the team's role in the murders of three unarmed civilians, but told Army investigators that his unit's "crazy" sergeant had hatched the plan.

READ: Confession Video: U.S. Soldier Describes Thrill Kill of Innocent Afghans

Morlock, described how Gibbs had the men in his unit pick out civilians at random and then kill them with grenades and rifle fire.

"He just really doesn't have any problems with f---ing killing these people," Morlock said, and then laid out the scenario he said the sergeant used to make it seem the civilians were killed in action.

"And so we identify a guy. Gibbs makes a comment, like, you know, you guys wanna wax this guy or what?" Morlock told investigators.

"Gibbs called it like, 'Hey you guys wanna, you guys wanna wax this guy or what?' And you know, he set it up, like, he grabbed the dude."

Morlock said that killing people came "too easy" to Gibbs. "He just really doesn't have any problems with f---ing killing these, these people, to be honest."

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