Mr. Sepi did not like to venture outside too late. But, plagued by nightmares about an Iraqi civilian killed by his unit, he often needed alcohol to fall asleep. And so it was that night, when, seized by a gut feeling of lurking danger, he slid a trench coat over his slight frame — and tucked an assault rifle inside it.

“Matthew knew he shouldn’t be taking his AK-47 to the 7-Eleven,” Detective Laura Andersen said, “but he was scared to death in that neighborhood, he was military trained and, in his mind, he needed the weapon to protect himself.”