Col. Steven McCraw of the Texas Department of Public Safety told reporters that officers made the "wrong decision" by waiting over 40 minutes before storming the classroom at Robb Elementary School, where an 18-year-old gunman had barricaded himself.

"Where I'm sitting now, of course, it was not the right decision. It was the wrong decision, period. There's no excuse for that. But again, I wasn't there," McCraw said.

McCraw explained that during an active shooter situation, the "rules change" and suggested that the officers should have acted much sooner to stop the massacre, which left 19 children and two adults dead.

"We believe there should have been an entry as soon as you can," McCraw added. "When there's an active shooter, the rules change."

McCraw also provided an updated timeline on the entire incident. He said that video evidence showed a teacher propping open the door that the gunman used to enter the building at 11:27 a.m. One minute later, the shooter crashed his vehicle into a ditch and opened fire on two men who went to investigate.

At 11:30 a.m., the teacher called 911. One minute later, the shooter approached the building and started shooting at windows around the school just as officers arrived.

McCraw noted that the school resource officer was not at the scene but rushed to the area after he heard the 911 call. As he made his way to the school, he drove past the suspect who was hiding by a vehicle.

The gunman entered the building at 11:40 a.m. and made his way to one of the classrooms inside. When officers responded four minutes later, they came under fire and retreated back outside to wait for a tactical team, which did not arrive until 12:57 p.m. Once on scene, they stormed the school and fatally shot the gunman.

McCraw also provided details about numerous 911 calls made from a girl inside the classroom. McCraw did not identify the student and did not release the audio of the calls.

The first call came in at 12:03 p.m. and lasted over one minute. The girl said she was in room 112. She called back seven minutes later and said that multiple people were dead. She called back at 12:13 p.m. and again at 12:16 p.m., telling the dispatcher that eight to nine students were still alive.

McCraw said that at 12:36 p.m., two or three shots could be heard. When the girl called back, the dispatcher asked her to stay on the line and told her to be "very quiet." At 12:51 p.m., McCraw said the call got "very loud" as officers started to evacuate the children from the classroom.

McCraw told reporters that investigators recovered 58 magazines and that the gunman fired over 100 rounds.

Bill Cunningham