The coverup predates that incident.
Correct—three earlier incidents in the 1940s:
Cape Girardeau—1941
https://www.amazon.com/Cape-Girardeau-1941-UFO-Incident/dp/1506034616
The Battle of LA—1942:
https://en.wikipedia.org/wiki/Battle_of_Los_Angeles
Trinity—1945:
https://www.amazon.com/TRINITY-Best-Kept-Jacques-F-Vall%C3%A9e/dp/B094ZQ1GW5