I doubt we’ll ever know the truth. Just like Area 51...it’s all secret.
Area may be secret, but lots of folks know what's there, or at least some of what's there.
I happen to have been assigned to the organization that once ran "Project Blue Book", although I never made it to the hanger, which did belong to them, where the remains of the aliens were said to be kept. Of course there was a sign on the bulletin board that they'd been moved, to the basement of the HQ building.
The building is built on a slab foundation.