Two things are nagging me. JH said the reenactments are inaccurate, and L's phone was under her body while her clothing was in the creek.
I think RA took them into the woods below the bridge on the south side of the creek, and that's where they spent nearly an hour, where he made them undress, etc. Then at 3:11, L's dad called, and whether or not the phone was on silent, maybe the phone lit up and L said her dad was there looking for them. With this, RA wanted to get away from the bridge and trails, so he made the girls gather their things and cross the creek. At 3:14, they might have been around the north bank when L's dad called the second time. I think the clothing was dropped and L maybe tried to get up the bank and answer the call. Maybe RA scrambled up behind her, getting muddy, they got about 50 feet from the bank before he chambered a second round to threaten her (or misfired and tried again), which ejected the round. Then he grabbed his knife and got to L, who fell on top of her phone. Or, alternatively, he killed L and while standing by her, chambered the round to threaten A to keep her from trying to run. JMO.