We're not solving any crimes here, but we sure are trying to figure out what happened. If the cyclist's tip is related, then this is most likely what happened: they went down the trail to meet someone, they parked their bikes, willingly or unwillingly they went over the fence, they walked into the forest towards the building and that was that.
If the cyclist's tip is valid, the girls probably went there to meet someone, thinking that they'd be home in time for 2 pm with Grandma. I doubt they willingly went over the fence because Elizabeth's purse/cell was dropped 10-20 feet to the east of the bikes, suggesting that she was hanging on to it until after she went over the fence and walked 20-30 feet.
If the cyclist's tip is not related, then anything is possible.