JOINT BASE LEWIS-McCHORD, Wash. — The soldier accused of being the ringleader of a rogue Army unit that killed three Afghan civilians last year for sport, crimes that angered Afghan leaders and villagers and rattled high levels of the American military, was found guilty of all charges on Thursday.

The soldier, Staff Sgt. Calvin Gibbs, 26, of Billings, Mont., was found guilty of three counts of murder, of conspiring to commit murder and several other charges, including assaulting a fellow soldier and taking fingers and a tooth from the dead. He was sentenced to life in prison but could be eligible for parole in less than 10 years.

The verdict, rendered in under a day of deliberations by a five-member panel after a nine-day court-martial at this base 45 miles south of Seattle, was a decisive victory for Army prosecutors, whose case against Sergeant Gibbs was built largely on testimony from other soldiers, including many who had pleaded guilty in the crimes. Of the five soldiers accused of murder in the case, three have pleaded guilty, one of them to manslaughter.

Sergeant Gibbs’s lawyer, Phillip Stackhouse, tried to convince the panel that most of the soldiers who accused his client were doing so to get more lenient sentences, and that accounts from the soldiers differed. Army prosecutors said that because many of the soldiers had already pleaded guilty to murder and other serious charges, they had no reason to lie. “All to frame Staff Sergeant Gibbs?” Maj. Robert Stelle asked the panel during his closing arguments on Wednesday. “It’s ridiculous. It’s ridiculous.”