In Wright Patterson base commander General Curtis Lemay's autobiography, he admitted, that some cases in Bluebook were unidentified & anomalous. That was in 1965 prior to Bluebook ending in 1969. New Govt documents from CIA, Navy, NASA & others are exposing the internal Govt struggle with what was coined, "The UFO Probelm."

"The bulk of them could be run down. Some natural phenomenon might usually account for those sightings which had been seen and reported, and thus explain them. However, we had a number of reports from reputable individuals (well-educated, serious-minded folks—scientists and fliers) who surely saw something.
There is no question about it: these were things which we could not tie in with any natural phenomena known to our investigators."
~General Curtis "Rand Corp" Lemay.