The following pdf file contains all of the resized Project Blue Book documents below and is now housed on the NICAP site for security reasons.
April 18, 1962, Western U.S.
Between 8:15 and 8:19 P.M. ?
Flash, Boom over Utah
Skies in Ten States
End of article states:
"The thought occurrs that this object was a perfect high-altitude flare--illuminating most of a two-state area to a point of daylight brightness. It so happens that the two states receiving most of the light were Utah and Nevada, both locations of U.S. Missile and nuclear test sites."
The original doc from the Blue Book Archive is listed below:
Page ID (PID) MISC-PBB2-1057
Collection Other Official Microfilm
Roll Description Misc Blue Book Files Roll 2