I think the FBI is involved in this case for several reasons, including the possibility that the suspect is in law enforcement, a high-ranking local official, or the son of an LE official. I think this is one of the reasons they moved the tip line down to the FBI headquarters.
IMO, at last week's press conference the FBI was speaking to the girls' families. LE stressed to the do NOT be afraid to call in a tip, NO ONE will know.
Overnight Search Stopped - I cannot comprehend this decision. (Side note example - A few years ago while leaving my home, I accidentally hit the emergency fire, police button on my key fob. While driving, I received a call from my alarm company informing me my alarm was activated and the fire department dispatched. I immediately turned around and drove back to my home. The fire department arrived at the same time. I ran up to the door and told them everything was ok. I accidentally pressed the wrong button. They were very nice, but the captain said under their mandate, they must enter and do a quick visual. I opened the door, once they confirmed all was safe, they left. Same thing happened about a year later, this time I was too far away. Luckily, my neighbor had my spare key and opened the door so they could do a visual and confirm everyone was safe. [I deactivated the button on the key fob]).
My point is the fire department would not leave or stop searching until they had a visual that everyone was safe.
The first day the girls were missing, no one knew their condition. LE was in the search and RESCUE part of their mission, so why call off the search for the girls until morning?
Again, one family could have thought it was ok for LE to stop searching overnight, but it's hard to comprehend both families agreed to this decision. Were they pressured?