Rick Hunter
Celestial
I don't think they know what it is anymore than the public. That's pretty scary for the intelligence agencies that are supposed to be protecting us on this planet. Its not only that they don't know what it is they don't know how to stop it.
I think it is most probable that 'they' (USAF, CIA, FBI, etc.) don't know anymore about aliens than a well-studied Ufologist would. They are also probably sitting on quite a bit of evidence and testimony that they wish to keep hidden, perhaps for no other reason than if they can't figure it out then they damn sure don't want the Russians, Chinese, or others to figure it out. It wouldn't surprise me if the various agencies in possession of UFO evidence are each unaware of whether other agencies have any.