Some notes:
I believe that BG has something heavy hidden under his jacket that he's supporting with his right hand. There's a bulge there that sort of spills over his hand and it's odd that he has that hand positioned higher than the other. He also appears to lean a little bit to his left.
I would be willing to bet money that it's a Fox Racing logo we're seeing peeking out of the top of his jacket.
I think what some people forget about the bridge is that it's a dead end. It's the perfect place to trap somebody. This is what you find if you go all the way across:
There is no trail on the other side. If you walk out onto it, which is not something that a lot of people do owing to the danger, you're going to be turning around and coming back.
I believe our perpetrator had a plan, that he would wait within sight of the entrance to the bridge for a victim to start to cross and then follow once they were no longer visible from the main trail. Once cornered, he would force his victim off the south end of the bridge and into a secluded area where he would murder them. Further, the place where they would have hypothetically crossed the creek is conveniently out of view of the bridge and the hiking trails, which I don't think is a coincidence. He would probably have made the victims cross the creek on foot. It would be too risky to carry them across after the crime; it would take two trips. The alternative to this is marching the victims back across the bridge
toward potential witnesses, and I just don't think that's very plausible.
It's possible that the perpetrator discarded the victims' phones in the creek and that partial data was recovered from them. Remember that a journalist reported seeing clothes in the creek near where the bodies were found. That could be the reason photos were not released immediately and that video has not been released. Of course, it could also be that full audio or video is too disturbing to be released to the public. Even a second of full-motion video of BG may show him approaching the girls in a threatening manner that LE doesn't want shown on the news. Pure speculation, of course, but totally plausible given the available information.