Well, that would have to be a fairly fast kill and disposal.
Tammy was there at 2:45ish per her account when she spoke to the fisherman.
Let's run on the theory this person did nothing but just take them to the park and kill them. No sexual assault time added in.
They were apparently last seen between 12:30 and 1 in the lake area.
So let's go with the abduction of the earliest time possible (per LE statements) 12:30.
Now account for the time to get the girls into the vehicle and drive to the park.
The drive is about 36 minutes per google maps. Let's go with 30 mins minimum to get to the park...probably longer, but let's do this on the fast side to be safe.
So that would place them at Seven Bridges at 1:00. Now, get the girls out of the vehicle...and walk them back to the site. That's probably 10-15 minutes minimum.
Now you're at 1:15 and at the site. How long does it take to kill two girls? Seconds if you use a gun. Minutes, if strangulation. So let's say 15 minutes minimum.
Now you're at 1:30 and headed back to the park to be seen by Tammy.
You've got 1hr 15 mins to get back to Meyer's Lake and it's a drive of approx 36 min.
Even if you take 30 mins to include a sexual assault at the scene there is still ample time to do it.
I just don't see that happening though.
I think this person took them somewhere ELSE and then dumped the bodies later at the park.
I just can't see the sexual assault (IF there was one) going down in a public area with 2 girls...
IF a sexual assault happened I believe without a doubt that they were taken somewhere, sexually assaulted and killed...and then they were taken to Seven Bridges later.