This PURSUE Release 01 document is a one-page FBI Detroit office memorandum dated April 17, 1958 and titled Unidentified Flying Objects. The release metadata identifies it as FBI material from Detroit, Michigan, and the official release page hosts the remote PDF.12
The memo was sent to the Special Agent in Charge in Detroit by Special Agent Robert Ross Reynolds. It records a same-night telephone report from David Weaver of Detroit, who called the FBI at 4:08 a.m. after unsuccessfully trying to reach Selfridge Field and the Air Force.2
Weaver reported seeing a circular object with a crystal-type dome that reflected lights, moving north from the southwest and crossing the city three blocks south of Six Mile at Lamphere Street. The memo says Weaver was 23, the son of a Detroit policeman, and had prior Civil Air Patrol experience, while noting that he was not very familiar with aircraft identification.2
The record matters because it preserves the origin and routing of a civilian UFO report rather than an official technical conclusion. The FBI documented the call, noted the witness's aircraft-identification caveat, and recommended advising proper Air Force authorities.2