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This US Air Force Project Blue Book case file on Kenneth Arnold's sighting of nine objects near Mount Rainier, Washington, on 24 June 1947 was released through The Black Vault. It contains the Project 10073 record card, which concludes the objects were a mirage, plus Arnold's telegram, questionnaire data and interview notes. It also includes J. Allen Hynek's analysis arguing the objects were closer and slower than Arnold estimated. Other items are David Johnson's sworn statement on a Boise sighting, a Tacoma investigation telegram and news clippings.
Read from the scan by GLM-OCR; expect the odd misread word.
Incident #17 -- page 3 original estimate is also incorrect; if the objects had travelled 47 miles in 102 seconds, they would have been travelling at a rate of approximately 1700 km/h, not 1200.) In view of the above, it appears probable that whatever ob- jects were observed were travelling at supersonic speeds and may, therefore have used some sort of known aircraft.
Document, cited by the archive. The PDF is mirrored here; the original link is above. The text was read from the page images by GLM-OCR; expect the odd misread word. 62 pages are in the text index: search them above, or from the library's search.