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…

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 vehicles from other worlds had occurred and that retrieved materials had been studied secretly for decades, often by aerospace companies under government contracts…

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.”…

Mr. Davis, who now works for Aerospace Corporation, a defense contractor, said he gave a classified briefing to a Defense Department agency as recently as March about retrievals from “off-world vehicles not made on this earth.”