Desirae Garza, whose niece Amerie Jo Garza was killed, said she rushed to the school when they heard the first reports of gunshots. Families being held back by the police outside the school grew increasingly frantic as they watched horrified students slowly escaping, being pulled out of windows and heading toward a nearby funeral home, some with blood on their faces, she said.

She said families were yelling at officers to do something, and pleaded that their children were still inside. When her brother Angel, who is Amerie’s stepfather, learned from a fleeing child that a girl named Amerie had been shot inside, he ran to try to reach his daughter but was handcuffed by a local police officer, Ms. Garza said.

It was nearly eight hours before he would learn that his daughter had been killed, she said.

“It felt like an eternity,” she said. “They weren’t letting any of us in anywhere. They kept pushing us farther and farther back in case he started shooting again.”