SEATTLE — The Army has denied early release for a soldier who tried to blow the whistle on a plot to kill Afghan civilians in 2010, only to later be convicted in the plot himself.

The clemency bid by former Spc. Adam Winfield of Cape Coral, Fla., was denied earlier this month despite the recommendation of a prosecutor who said he deserved credit for initially trying to stop the killings and for his cooperation in testifying against the plot’s ringleader, Staff Sgt. Calvin Gibbs of Billings, Mont.

“Certainly, Spc. Winfield does not have completely ‘clean hands’ when it comes to these events,” the prosecutor, Maj. Robert C. Stelle, wrote in a memo. “However, it is also true (and well documented) that he attempted to blow the whistle.”

Winfield also confronted Gibbs about his conduct, which led to a series of threats being made against him to keep him quiet, the prosecutor said.

Stelle urged the commander of Joint Base Lewis-McChord, Maj. General Lloyd Miles, to “strongly consider granting Spc. Winfield some relief from the remainder of his jail sentence,” which is set to expire at the end of August.

Miles, however, denied clemency on April 5; he did not state a reason.

Winfield was one of four Washington state-based soldiers convicted in the deaths of three Afghans during patrols by his unit in Kandahar Province in January, February and May 2010. In each of the killings, the soldiers involved found isolated Afghan men or teenage boys and staged the killings to appear to have been combat-related.

When Winfield learned of the first killing, he sent Facebook messages home to his father, who reported the matter to Joint Base Lewis-McChord the same day.

The soldier who took the call from Winfield’s father, Chris, later said he didn’t alert his superiors to the claims because there was no standard operating procedure for doing so. Instead, the soldier told Chris Winfield that he should encourage his son to come forward to authorities in Afghanistan — something Adam Winfield did not feel safe doing.

The Army did not arrest the plotters until May. By then, two more civilians were dead.

Winfield said he was pressured to participate in the third killing. He pleaded guilty last summer to a charge of involuntary manslaughter and was sentenced to three years.

Chris Winfield, a former Marine, said Wednesday that clemency “would have acknowledged that Adam was telling the truth from the beginning, that he tried to do the right thing.”

Winfield’s testimony helped convict Gibbs, who was sentenced to life in prison.

In all, 11 soldiers in the unit were convicted in connection with misconduct that, in addition to murder, included hash smoking, collection of illicit weapons, the mutilation and photography of Afghan remains, and the gang-beating of a soldier who reported the drug use and unjustified killings.

Winfield’s confinement on base has recently included a regular job working at a greenhouse, where he grows vegetables, his father said.