Investigations
Sign's first major investigation occurred in the aftermath of the widely-publicized Mantell Incident. On 7 January 1948, Air Force pilot Capt. Thomas Mantell crashed his aircraft near Franklin, Kentucky while in pursuit of a UFO. Numerous eyewitnesses, both civilian and military, had reported a large, metallic object in the skies. Mantell was one of four pilots on a nearby training mission who were ordered to investigate. Upon reaching about 10,000 feet, Mantell's companions abandoned their pursuit due to a lack of high-altitude oxygen gear. Mantell continued, however, and reportedly described the aerial artifact as "a metallic object ... it is of tremendous size." Mantell is presumed to have blacked out from oxygen deprivation at about 15,000 feet, whereupon his airplane crashed and he died. The USAF formally announced that Mantell had died in pursuit of the planet Venus. Sign's personnel never accepted this explanation, and regarded the incident as an unknown. Some believe that Mantell died while chasing a then-secret Skyhook high-altitude weather balloon.
Other investigations followed the Mantell case. On the evening of 18 February 1948, an unusual light illuminated the skies over Norcatur, Kansas. An accompanying shockwave broke windows, and area residents initially thought an airplane had exploded in flight. Sign did not formally investigate, but consulted with scientist Lincon La Paz. The incident was probably an exceptionally bright bolide, said La Paz. But his explanation was provisional as no fragments were discovered and some eyewitnesses testimony was inconsistent with a meteor.
On 5 April 1948, three experienced balloon technicians at Holloman Air Force Base in New Mexico reported an unusual sighting of two roughly circular objects, white in color and very high in altitude, moving erratically and at great speed. The technicians asserted to Loedding that the objects were definitely not balloons or conventional aircraft. Moreover, the witnesses also reported that odd radar returns were common at Holloman, distinct from the usual radar "angels" caused by clouds or other known phenomena. The Holloman incident was reported as an "unknown" in Sign's files.
On 7 May 1948, two witnesses near Memphis, Tennessee claimed to have observed 50-60 silvery objects moving at high altitude and in tight formations. Sign reached no conclusion on this incident, but later Air Force records describe the objects as meteors. However, astronomer Paul Herget had earlier specifically excluded this explanation. This case was the first to feature the involvement of J. Allen Hynek, who is believed to have invented the meteor explanation. Hynek, then teaching astronomy at Ohio State University, was hired as a consultant to help weed out UFO reports which could be misidentified meteors, stars and the like. He would consult not only for Sign, but for Project Grudge and Project Blue Book until the latter was decommissioned in 1969. In a 1985 interview, Hynek reported that he was initially dismissive of UFO reports: "I was quite negative in most of my evaluations. I stretched far to give something a natural explanation, sometimes when it may not have really had it." By the mid-1960s, however, Hynek's opinion had changed: he believed, after encountering a minority of UFO reports, that, in his opinion, some reports seemed to defy conventional explanation. Hynek also spoke out against what he saw as shoddy research by the Air Force.
On 17 May, businessman William A. Bonneville reported an extended UFO incident while driving between Plevna, Montana and Miles City, Montana. A luminous ball, brilliantly lit and brighter than the moon, emerged from behind hills to the northwest and traveled first to the south, then to the west, over a 20 minute period. Sign regarded this as an unknown but later files, possibly Hynek's interpretation, blamed the incident on "refraction of the planet Mars."
Near Monroe, Michigan on 25 May, two Air Force officers reported a UFO sighting. They were passengers in an Air Force plane when one officer observed three disk-like objects flying at about the plane's altitude for 10–15 seconds before making a sudden sharp turn and accelerating rapidly out of view. The officer spoke to his traveling companion, who hadn't observed the incident. Moments later, however, both officers reported the appearance of two objects, similar to the first grouping, which maneuvered radically and accelerated rapidly into the distance. Sign classified this incident as an unknown.
A husband and wife driving near Hecla, South Dakota reported another sighting which Sign classified as an "unknown." On the evening of June 30, the couple spotted what they took to be an unusually bright star. The husband, a professional engineer and amateur astronomer, stopped their car several times to observe the light, which he eventually realized was not a star. Three small glowing fragments seemed to "fall off" the light. The three fragments arranged themselves in the points of an equilateral triangle around the light. The triangular formation maintained its geometric proportions as it rose to a great height and disappeared from view. The engineer would later assert, "my convictions at this point were that it could not have been anything terrestrial."
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