My theory is that they had it ‘rewound’ so they could watch the ‘suspect’ enter the building and start shooting, so that they could identify the ‘shooter’.
By the time the POLICE arrived at the school, all the shooting was over and the ‘suspect’ was on his way to SUBWAY.
The POLICE saying they didn’t know it was TIME DELAYED seems to be an excuse for why they (or the Deputies) didn’t enter the building when the shooting first started.
I thought the shooter stopped at McDonalds?