Really? Give me a counter example that illustrates your point.
For the six years that the public knew there was a stealth bomber (or was it the stealth fighter) they simply said “we don’t know what you’re seeing. We don’t have anything like that.” No contradictory and stupid “we don’t know what it is but it’s no threat” story. I can’t remember any time they ever said they were investigating something like this just to come out a few days later and say “we think it was an airplane contrail but we don’t know.”