Timeline again.
I reread the article and it says Authorities were notified at 5:40 p.m. I originally thought that this meant the 911 call occurred then, but now I am thinking that is when 911 people contacted the sheriff's department to go to the scene.
4:55 - 5:00 Girls leave house. (assumed)
5:08 - 5:10 Skyla's mother calls (reported in interview)
5:13 - 5:25 Sometime in this time frame, Grandfather attempts to call girls to tell them to return (assumed)
5:10 - 5:25 Witnesses reported seeing girls walking and the suspicious person (don't know if they are the same witness or two different ones) (assumed since girls are seen alive)
5:20 - 5:25 Assumed time of death (reports say sexual assault unlikely because whole event takes place in 20-25 minutes)
5:25 - 5:30 Grandfather leaves on foot to look for girls (assumption because he is going north toward the girls and doesn't see white pickup in the road (not reported at least,) so murder has already occurred)
5:30 - Grandfather finds girls, calls 911
5:40 Sheriff's office contacted
Past 5:30 is when Skyla's mother arrives.
From all the witnesses and articles, it seems they have a pretty good timeline of when all this occurred, but they haven't released it yet. There's a reason for that: either because it will help them catch the suspect, or they are protecting witnesses.
It sounds like the witness who either saw the girls or who helped with the sketch can be easily identified if certain things are released (due to schedule, etc.) and/or they live REALLY close by.