The gunman walks into Robb Elementary School unimpeded, moments after spraying bullets from his semi-automatic rifle outside the school and after desperate calls to 911 from inside and outside the school.

He slows down to peek around a corner in a hallway and flips back his hair before proceeding toward classrooms 111 and 112.

Seconds later, a boy with neatly combed hair and glasses exits a bathroom to head back to his class. As he turns the corner, he notices the gunman standing by the classroom door and unloading a barrage.

The boy turns and runs back into the bathroom.

The gunman enters one of the classrooms. Children scream. The gunfire continues, stops, then starts again. Stops, then starts again. And again. And again.

It is almost three minutes before three police officers arrive in the hallway and rush toward the classrooms, crouching down. Then, a burst of gunfire. One officer grabs the back of his head. They quickly retreat to the end of the hallway, below a school surveillance camera.