-UFO crashed for unknown reason
-Military cleaned it up, recovered materials and silenced the civilians
-Military changed their story from "saucer" to "weather balloon"
(After the panic that War of the Worlds caused, I don't blame the military for keeping this quiet).
-Military tried to keep info away from the KGB, etc.
-New technologies surfaced in the market due to reverse engineering of the crash materials.
Some of these technologies include: night vision, lasers, kevlar, and integrated circuits.
Possible truths or lies, who knows. It's still very interesting, though.
The Day After Roswell by Philip Corso
You guys should read it