"Staff Sergeant Wuterich accepted responsibility ... and agreed and admits that he gave a verbal order to shoot first, ask questions later, or don't hesitate to shoot, and words to that effect," said spokesman Joe Koppel.
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"That verbal instruction caused his marines to [not] positively identify targets in the two homes. And now, at the sentencing phase, he'll be held accountable for those actions."
The victims included 10 women and children killed at point-blank range. Six people were killed in one house, most shot in the head, including women and children huddled in a bedroom.
The other seven soldiers charged in the case had been exonerated through various legal rulings, fuelling anger in Iraq, where authorities had pushed for US troops to be subject to Iraqi justice before the US pullout in December.
Wuterich now faces a maximum sentence of three months of confinement, forfeiture of two-thirds of his pay for three months and a reduction in rank when he is sentenced, a base spokesman said.