following pdf file contains all of the resized Project Blue Book
documents below and is now housed on the NICAP site for security
http://www.nicap.org/docs/hanford_let500804doc.pdf Memorandum For The Record (actual copy)
This MFR looks like the Army (propably the Army Anti-Aircraft unit higher hqs or if there were additional units assigned Hanford. An LTC as G3 indicates a higher HQ. Or this is a memo at some staff level like 6th Army or possibly even in the Pentagon.)
The AEC records would be of interest. The DOE archives in Nevada supposedly have no hq security and intelligence files. Yeah, sure. I don't think that Hanford has been looked at like Los Alamos or Oakridge as far as FOIAs are concerned.
FBI records for Hanford probably have contain this incident. The Seattle office did do a search for UFO items for Todd, but again anything for Hanford might be in a separate file.