I am completely chilled hearing that and knowing there is more video. I wonder if when he/they found she had her phone on he/they threw the phone away from her and the video kept rolling? Did she record it on snapchat and if so did anybody actually see it?
As far as SnapChat, the videos and images only last for 10 seconds each (filming wise) - and if it had gone out over SC anyone who had access to her SC her feed could have seen the image(s)/video for the time it stays up.
I'm sure we'd have seen long ago from someone if there were more relevant SC images/video because they'd have been found or shared immediately.
I believe what we're seeing of the suspect (and hearing audio wise) came from regular video and/or audio that Liberty managed to capture (I suspect she probably got her phone into a pocket or bag or something at some point which is how they got as much as they seem to have).
But SnapChat wouldn't have recorded enough, would have been found by others, and she'd have had to have kept stopping and starting recording at least every 10 seconds basically.