An investigation becomes an archive
The U.S. Air Force’s UFO investigations left a substantial paper trail. According to the National Archives, 12,618 sightings were reported between 1947 and 1969, with 701 categorized as unidentified. The investigation records are now available for historical research.
What the conclusion said
The Air Force stated that its investigation found no evidence that the unidentified sightings were extraterrestrial vehicles or represented technology beyond contemporary scientific knowledge. Project Blue Book was terminated in 1969.
What unidentified leaves open
An unresolved entry is a statement about available information. It is not a positive identification of something extraordinary. Missing measurements, conflicting accounts, and incomplete records can all limit what a researcher can conclude. The most useful way into Blue Book is through individual files and their reasoning, rather than treating one total as proof.