SEATTLE — Staff Sgt. Robert Bales of the Army, who is accused of killing 16 Afghan civilians last year, most of them women and children, is expected to offer on Wednesday his own account of the attack for the first time, under oath, as part of a guilty plea hearing that would remove the threat of the death penalty from his case.

Sergeant Bales, 39, would instead face life in prison if the guilty plea is accepted by the military court at Joint Base Lewis-McChord, about an hour south of Seattle, where he was stationed.