The Roswell Saucer, before its crash
The Project Saucer Report of April 1949 gave its explanation for saucers seen by Army Air Force pilots and ground officers over Muroc Air Base in California (now Edwards Air Force Base) on July 8th, 1947: Research Balloons.
This is from Project 1947's account, taken from Richard Hall's The UFO Evidence:
At Muroc AFB (now Edwards AFB) and adjacent Rogers Dry Lake, scientists and engineers test and develop the latest aircraft, including secret projects. Althoroughly [sic] familiar with anything that flies, the base technical personnel had no explanation for the UFOs which maneuvered over the area July 8, 1947. Twice that morning, disc-shaped objects were observed cavorting overhead. Then about 11:50 AM, a crew of technicians at Rogers saw a round white, apparently metallic object descending, moving west nortwest [sic] against the wind. They observed thick projections on top which crossed each other at intervals, suggesting either rotation or oscillation.
One of the Muroc saucers is that which crashed near Roswell, and from which slides of the recovered bodies exist.
Click HERE for the Project 1947 documents about the Muroc sighting.