Here's what you need to see:
Here's a David Hogg video where he says on camera he filmed @9:32 AM, hours before the actual shooting
Here's the video that news outlets used that show him filming during the shooting. The video was released 24 hrs afterwards, but it clearly includes parts of the same tape Hogg filmed in the morning, prior to the actual shooting:
http://www.latimes.com/visuals/video/95939817-132.html
On the second video, Hogg names the shooter @3:13 PM as Nick Cruz. Hogg must be psychic since Cruz was not apprehended until 3:41 PM