All told, seven people died in connection with the U.S. Capitol riot on Jan. 6. But only Ashli Babbitt’s death was directly caused by violence that day. She was a rioter killed by a Capitol Police officer, who fired the only shot by any person during the 4½-hour siege. Yet the story of who he is and why he opened fire remains shrouded in mystery.More than six weeks after Babbitt succumbed to a single gunshot wound to the upper chest, authorities are keeping secret the identity of the officer who fired the fatal round. They won’t release his name, and...