Around the 4:30 mark there is a split screen of security footage of the outside door the shooter went in and a parking lot he was walking across and someone filming him walking into a door from a cell phone.

They circle the door in red and arrow him in red in the parking lot and you can see him moving in the parking lot but I never see him walking to the door circled and it doesn't seem like the cars/trees/sidewalks/grass are the same angles or arrangement as it looks in the cell phone footage. Imagining it from the side, it doesn't seem right. Plus I can't ever see him walking to the door and it cuts away as soon as he goes in the door. I don't think he enters the door that is circled. There is a door further in the back of the video which would be more likely he walked to and maybe the reason why you can't see him walking to the door circled?

There is also a truck parked on the street in front of the circled red door for awhile during the teacher's 911 phone call.

If the door was propped open you'd see light shinning in before he opens it. I can't remember what the narrative moved to if it was propped open or just didn't lock.