Laura Sandoval threaded her way through idling taxis and men selling bottles of water toward the entrance of the Cordova International Bridge, which links Ciudad Juárez, Mexico, to El Paso, Texas. Earlier that day, a bright Saturday in December 2012, Sandoval had crossed over to Juárez to console a friend whose wife had recently died. She had brought him a few items he had requested—eye drops, the chimichangas from Allsup’s he liked—and now that her care package had been delivered, she was in a hurry to get back to the Texas side, where she’d left her car. She had…