In one of the on-line news phone interviews with one of the wounded, one of the surviving witnesses said the shooter opened the classroom door, emptied the magazine, and then slammed the door.
In another, he reportedly lined his victims up in a row, then went down the line executing them.
My sister in law is an RN at Montgomery Regional and she had a patient tell her the same story. A girl being treated for a gunshot tro her hand said they shooter burst in the room and fired indiscriminately and then went to next class. The students barricaded the door and he came back...shooting into the door.