Investigators in Texas were seeking to determine on Saturday how critical mistakes were made in response to the deadly Uvalde shooting, including why nearly 20 police officers remained outside a school classroom as children made 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 an ongoing 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. Investigators are also still searching for a motive for the attack. Salvador Ramos, a high school dropout, had no criminal record and no history of mental illness, although threatening messages he sent on social media are now coming to light.