I doubt the craft was extra-terrestrial in origin. What I really think is that they encountered an advanced, un-piloted drone surveillence platform that wasn't functioning properly that had been sent into spy on the installation. No jet engines, no propellers, triagular, metallic skin, some lights on it. May or may not have left radioactive traces on the landing site. May have caused burn marks on the trees next to the landing site.
How about a craft powered by a nuclear source to provide electricity such as used for satellites? Remember the one that crashed in Canada a couple decades ago? And what if this particular craft used a Biefeld–Brown effect for levitation and propulsion? Discharges into the trees would cause burn marks. If I lost control of the craft, and noticed two enlisted guys were next to it. I'd probably wait to power it up only after they had gotten far enough away to take off without electrocuting them. And I'd fly that thing out of there as soon as possible to prevent it from falling into their hands permanently. Purely human activity with advanced technology not seen or understood by more than a handful of people at the time, being used for cold war spying on nuclear facilities. Yeah, sounds like a James Bond story, but a lot more beleiveable than ETs. AND definitely a threat to national security.