So in this video where police are removing something from the school there are people, or police, in the background at the entrance to the school building to the south. One women walks out in civilian clothes wearing a red or pink printed top. What we are looking at is the NE end of the freshman building. The SWAT vehicle is already there. There is not much time between when the woman walks out and the "bag" is carried out. Can't tell which one is first. I am guessing the woman walks out before because the other officers are basically in the same position. I pasted these two together for shadow comparisons and can't see any difference so not much time in between.
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I am guessing that this is about 10-15 mins into the event. If you watch the main video, 2hrs long by ABC, it closes in on the arrest of Cruz but at this point he has not been apprehended yet.
I don't think there is a body in the bag. Too stiff and not large enough. I think it is the weapon used as the teacher described here. Also notice at the beginning you can see that same truck now backed up to the door. That truck was parked in the parking lot before it was moved to take the "bag" and as you see who ever was in it probably ate his lunch there and decided to toss his trash out because he can...