Observation 1) There has never been any threat by the girl or there would be PC to arrest. Threatening that kind of action, is an offense that gives PC to arrest. As is even threatening to harm one person.
Observation 2) It's kind of unusual to call the parents and have an "FBI Investigator" answer the parents phone, respond to the media, and claim to be questioning the parents.

To the extent this is real, it's boiled down to:
1. 18 yo girl might have expressed interest in Columbine (but has apparently never threatened to do a repeat, which would be PC)
2. 18yo girl might have traveled to Colorado and might have bought a shotgun.

Meanwhile, all of the front range panics like an ICBM is incoming, shutters all the schools, etc.

I wonder to what extent this is actually an NSA case -- you know, how they give "tips" to local law enforcement based on all the surveillance of US Citizens that they don't do, giving LE a heads up to invent PC to arrest someone the NSA feels is dangerous, but yet, can't be based upon what the NSA actually intercepted because that would be, illegal to spy on it's own citizens without PC.