BBM
My opinion is that the FBI is NOT obligated to tell us anything during an active investigation. This again is why I think the statement that they believe they are alive is so unusual. My sense is that this is a huge investigation, involving lots of individuals who have played a part in some sort of criminal activity. The disappearance of these girls is one facet of the criminal enterprise target, and just might have included child trafficking. I think they may have been handed off to a series of individuals, and at some point the FBI was able to get them to a safe house of sorts to protect them from further harm while they track down all the folks who had anything to do with it. In this scenario, I believe they would not necessarily tell the family anything other than they believe the girls are alive because some members of the family might also be a part of the criminal enterprise. And they sure wouldn't tell the public because that could compromise the identity of any confidential informant that helped get the girls to the safe place.
Possible? Probable?