Richard Hall:
October 9, 1951: An earlier UFO, rated an "unknown" by the Air Force after investigation of similar evidence (apparently without radar confirmation) was reported at Municipal Airport, Terre Haute, Indiana. About 1:43 p.m., CAA Airways Operations Specialist R. L. Messmore noticed an unusual object approaching from the SE, and quickly called another witness. C W. Sonner, Chief of Interstate Airways Communication Station, ran outside to watch. "I have been working at airports for 16 years." Sonner said, "and never before have I seen an aircraft like it." The flattened round object sped overhead, disappearing to the NW after 15 seconds. Using the angle of sighting, Messmore and Sonner calculated that the UFO was traveling at 2,880 mph, assuming it was at treetop level; 18,000 mph if at 3,000 feet; etc. 

Because of the experience of the observers, this would have been a good sighting as it stood. But two minutes later, near Paris, Illinois (19 miles to the NW), a private pilot encountered a hovering UFO shaped like a flattened sphere. (See diagram.) When the pilot turned directly toward the UFO, it accelerated and shot away to the NE. 

Source: The UFO Evidence, page 44