Mr. Elizondo is among a small group of former government officials and scientists with security clearances who,
without presenting physical proof, say they are convinced that objects of undetermined origin have crashed on earth with materials retrieved for study.
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In some cases, earthly explanations have been found for previously unexplained incidents. Even lacking a plausible terrestrial explanation does not make an extraterrestrial one the most likely, astrophysicists say.
Mr. Reid, the former Democratic senator from Nevada who pushed for funding the earlier
U.F.O. program when he was the majority leader, said he believed that crashes of objects of unknown origin may have occurred and that retrieved materials should be studied.
"After looking into this, I came to the conclusion that there were reports — some were substantive, some not so substantive — that there were actual materials that the government and the private sector had in their possession," Mr. Reid said in an interview.
No crash artifacts have been publicly produced for independent verification. Some retrieved objects, such as unusual metallic fragments, were later identified from laboratory studies as man-made.
Eric W. Davis, an astrophysicist who worked as a subcontractor and then a consultant for the
Pentagon U.F.O. program since 2007, said that, in some cases, examination of the materials had so far failed to determine their source and led him to conclude, "We couldn't make it ourselves."