"Bertrand also noted that their UFO sighting took place nearly an hour after Operation Big Blast was said to have ended, which eliminated the operation as a possible cause of the sighting.When Project Blue Book did not respond to their letter, on December 29, 1965 — nearly four months after the sighting — the two men sent another letter to Blue Book in which they wrote that the object they observed "was absolutely silent with no rush of air from jets or chopper blades whatsoever.