Sergeant Robert Bales, 39, pleaded guilty to 16 counts of murder over the massacre in southern Afghanistan in March last year. His lawyer said last week he would admit guilt in exchange for prosecutors not seeking his execution.
Bales was flanked by her lawyers John Browne and Emma Scanlan in a packed courtroom at Joint Base Lewis-McChord south of Seattle in Washington state, where he has been held pending court martial proceedings.
Scanlan entered guilty pleas for all charges against him including six of attempted murder and seven of assault. Seventeen of the 22 victims were women or children and almost all were shot in the head.
When asked by military judge Colonel Jeffery Nance if he understood that a guilty plea was final, Bales responded crisply, "Yes, sir."
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A date of August 19 was set for a sentencing trial. Bales requested trial by 12-member jury including one-third enlisted officers. Prosecutors did not immediately confirm that they were no longer seeking the death sentence.
In death penalty cases, a unanimous verdict is required.
Bales' lawyer John Browne announced last week that he had reached "an agreement with the military to take the death penalty off the table" if Bales would plead guilty.
Bales allegedly left his base in the Panjwayi district of southern Kandahar province on the night of March 11, 2012, to commit the killings. Nine of those killed were children. Bales allegedly set several of the bodies on fire.
At a pre-trial hearing in November, Bales's family insisted he was innocent until proven guilty, calling him "courageous and honorable," while his lawyer raised questions about the role of alcohol, drugs and stress in the tragedy.
Prosecutors at the so-called Article 32 pre-trial hearing alleged that Bales left the base twice to carry out the killings, returning in between and even telling a colleague what he had done.
The hearing included three evening sessions -- daytime in Afghanistan -- to hear testimony by video conference from Afghan victims and relatives of those who died.