Police have dramatically changed their account of how the Las Vegas massacre began on Oct. 1, revealing Monday that the gunman shot a hotel security guard six minutes before opening fire on a country music concert — raising new questions about why police weren’t able to pinpoint the gunman’s location sooner. Officials had previously said that gunman Stephen Paddock, 64, of Mesquite, Nev., shot Mandalay Bay security guard Jesus Campos after Paddock had unleashed his deadly volley at the Route 91 Harvest festival, an assault that began at 10:05 p.m. and left 58 people dead, with hundreds more injured. They...