Aside from drones, all engines rely on burning fuel to generate power, but these vehicles all had no air intake, no wind, and no exhaust.

…"Imagine you see highly advanced vehicles, they appear on radar systems, they look bizarre, no one knows where they're from. This happens on a recurring basis, and no one does anything," said Mellon, who now works with UFODATA, a private organization.

"It's very mysterious, and they still seem to exceed our aircraft in speed," he said, calling it a "truly radical technology."

In 2017, the Pentagon first confirmed the existence of the Advanced Aerospace Threat Identification Program, a government operation launched in 2007 to collect and analyze “anomalous aerospace threats.” As the Post’s Joby Warrick reported, the investigation ranged from “advanced aircraft fielded by traditional U.S. adversaries to commercial drones to possible alien encounters.”

https://www.philly.com/news/nation-world/unidentified-flying-objects-ufos-navy-document-encounters-20190424.html?outputType=amp