The Pentagon has admitted it spent tens of millions of dollars on a secret UFO investigation branch for five years.
The Advanced Aerospace Threat Identification Programme (AATIP) ran between 2007 and 2012, but the New York Times reports some of its staff are still investigating strange lights and phenomena, even more quietly than before.
The AATIP was set up by former Democratic Senator for Nevada, Harry Reid. Nevada is home to Area 51 - a top-secret US Air Force base which some UFO conspiracy theorists believe houses a crashed alien spacecraft.
"I'm not embarrassed or ashamed or sorry I got this thing going," Mr Reid told the Times.
"I think it's one of the good things I did in my congressional service. I've done something that no one has done before."
Few officials were even aware of the programme, and its existence has never been publicly acknowledged until now.
Documents show the AATIP investigated sightings of aircraft that appeared to have no visible signs of propulsion or lift.
Experts doubted the shadowy unit would have uncovered anything new.
"There are plenty of prosaic events and human perceptual traits that can account for these stories," ex-shuttle engineer James Oberg told the Times.
"Lots of people are active in the air and don't want others to know about it. They are happy to lurk unrecognised in the noise, or even to stir it up as camouflage."
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The AATIP lost its funding in 2012 when Department of Defense officials decided there were better ways to spend taxpayers' money.
Its head, military intelligence official Luis Elizondo, quit his official Pentagon role in October. In his resignation letter, he questioned why the US wasn't spending "more time and effort" on UFO research.