I don't believe extraterrestrial spacecraft have ever visited Earth.
I'm not aware of any instance where extraterrestrial technology was used in an aircraft or to push the envelope for military flight technology.
In fact, much of it has a publicly-traceable lineage, if people would just dig. I'm thinking of low observability (stealth) technology, the latest RADAR/LIDAR tech, airframe designs, ram/pulse/pulse detonation jets, and so on. I admit some of it looks like magic on first glance, but this stuff was designed by human beings who were much smarter than me.
At the risk of wading into a pointless flame war, let's apply Occam's Razor.
First, public belief/interest in alien flying saucers has been mostly dead for decades. Performing a cover-up operation now would accomplish nothing.
Second, if the original flying saucer sightings truly were of extraterrestrial spacecraft (which I highly doubt) then nothing in these documents precludes these designs being based on those spacecraft. So, I expect this release to fuel the flames of the conspiracy theories, not cool them down.
