The loud, thunderous noise was widely reported Sunday throughout the D.C. region -- including neighboring states -- and it was later determined that the whole thing was planned by our armed forces, which was revealed later in the day ... despite spooking everyone.

At first, it seemed like it was just a drill they were conducting ... but now, the word is these F-16s were actually scrambled in an effort to intercept a small aircraft that was flying where it wasn't authorized to ... and which was apparently not responding to ATC signals.

Reports say the plane, a Cessna, lost altitude quickly -- and because no one knew where it came from or why it was cruising so close to the WH ... the military sprung into action.

ABC is reporting that the F16s actually caught up to the plane while it was still in the air -- but while it was also going down -- and that they actually saw the pilot of Cessna passed out inside. The plane ultimately crashed, and 4 people are said to have been onboard.

Their identities and conditions are unknown at this point, as is the cause of the plane going down in the first place. The FAA says the Cessna was going from Tennessee to Long Island -- but once it was in NY, it inexplicably turned around and flew in a straight line for D.C.