By Brad Brooks and Gabriella Borter UVALDE, Texas (Reuters) -Investigators in Texas on Saturday sought to determine how critical mistakes were made in the response to the deadly Uvalde school shooting, including why nearly 20 officers remained outside a classroom as children placed panicked 911 calls for help. Why the officers waited in the hallway nearly an hour before entering and fatally shooting the gunman is at the heart of a probe by the Texas Department of Public Safety into the massacre of 19 children and two teachers in the deadliest U.S. school shooting in nearly a decade. With calls…