RIO DE JANEIRO — Eduardo de Jesus was on his doorstep in Complexo do Alemão, a vast maze here of cinder block homes, when his mother heard the loud blast of gunfire.

Seconds later, she saw Eduardo, 10, lying dead from a gunshot wound to the head, and she ran toward the police officer holding the gun.

“I grabbed him by the vest and yelled, ‘You killed my boy, you wretch,’ ” said his mother, Terezinha Maria de Jesus, 40.

“He told me, ‘Just as I killed your son, I can kill you, too,’ as he pointed his rifle at my head,” she continued. “I told him: ‘Go ahead. You just killed part of me. Take the rest.’ ”