JOINT BASE LEWIS-McCHORD, Wash. — Staff Sgt. Robert Bales, the enigmatic figure at the center of the worst American war crime in recent memory, admitted for the first time on Wednesday deliberately killing 16 Afghan civilians last year, most of them women and children.

He took the oath in a military court, swore to tell the truth, and conceded in crisp “yes sirs” and “no sirs” every major charge against him — that he shot some victims, and shot and burned others, and did so with complete awareness that he was acting on his own, without compunction or mercy or under orders by a superior Army officer. The guilty plea removes the possibility of the death penalty in the case.

But the curtain of enigma about the man himself, and his descent into darkness and murder on the night of the killings, remained firmly in place. The millions of Americans who have pondered the mechanisms of atrocity since the attacks in March 2012 were left in the dark. Even Sergeant Bales himself, finally pressed by the presiding judge, Col. Jeffery Nance, to explain more deeply what happened, seemed baffled.

“What was your reason for killing them?” Colonel Nance finally asked.

Sergeant Bales, 39, seated at the defense table in his blue service uniform, hands clasped before him — thumbs often nervously twitching — said he had asked himself the same question “a million times.”