On a Saturday morning in August 2019, a 21-year-old White man with ear protectors, safety glasses and an AK-47-style rifle walked into a crowded Walmart in El Paso, Tex., his pockets bulging with ammunition. He had to driven hundreds of miles across the state, prosecutors say, because he wanted to kill Latinos.Kevin McAleenan, the acting Homeland Security secretary, was at a Coast Guard picnic in Virginia that day, and soon the urgent messages began arriving. A sinking feeling of horror set in as the magnitude of the attack became clear. “It was devastating,” he said.Twenty-three people were murdered in the...