It was reported Cruz ran out with other students. He was wearing a MSD shirt. Not sure how they would have distinguished him from hundreds of other students.
In a perfect world, all the students who came out of the building would have been directed to an enclosed/cordoned-off area, to be contained until they could be sorted through and individually questioned. I’ve tried to find out if Cruz had dumped his weapons before leaving the school, but couldn’t find any clarification on that issue.