The U.S. Congress is set to hold its first public hearing on UFOs since the 1960s on Tuesday (May 17), drawing on defense intelligence testimony to establish what could be causing the phenomena.

The hearing — which will question two Pentagon experts on what they know about UFO sightings — will focus on the contents of a June 2021 Pentagon report that revealed navy pilots had reported 144 sightings of unidentified aerial phenomena (UAPs) since 2004, most of which the department concluded "probably do represent physical objects."

Of these reported UAP sightings, 18 displayed extremely unusual flight behaviors, with the mysterious objects appearing to "remain stationary in winds aloft, move against the wind, maneuver abruptly or move at considerable speed, without discernible means of propulsion," according to the report.

Related: 9 things we learned about aliens in 2021

Released video clips show some of these ostensibly propulsionless craft moving at hypersonic speeds, Live Science previously reported, and one piece of footage (captured by the U.S. Navy) appears to show a spherical UFO hovering in midair while bouncing from side to side, before plunging into the ocean. A former Pentagon official, Luis Elizondo, also told the Washington Post that UAPs had interfered with secret U.S. nuclear weapons facilities, even forcing some offline.

Congressional oversight committees have been investigating the claims since 2017 after Politico and the New York Times dropped a series of bombshell reports on the Pentagon's secret UFO research office, and the testimonies of the Navy pilots and radar crew who had encountered the strange aerial objects. Now, a public hearing is imminent.

"Congress hasn't held a public hearing on unidentified aerial phenomena (UFO's) in over 50 years," meeting Chair and Indiana Representative Andre Carson wrote on Twitter. "That will change next week when I lead a hearing in House Intelligence on this topic and the national security risk it poses. Americans need to know more about these unexplained occurrences."

At the public hearing — streaming live here from 9 a.m. EDT (1300 GMT) — U.S. representatives will question Undersecretary of Defense for Intelligence and Security Ronald Moultrie and Deputy Director of Naval Intelligence Scott Bray on what they know about the unexplained phenomena.

After this public hearing, Congress will hold a closed, classified hearing on the activity of the Pentagon group — known as the Airborne Object Identification and Management Synchronization Group (AOIMSG) — that has been tasked with uncovering the possible explanations for the sightings.

What the hearing will reveal isn't clear, but pertinent questions include whether UAPs could be satellites, belong to foreign governments or be purposeful fakes. If the experts still haven't ruled out an extraterrestrial explanation, representatives could also ask them to give further details on the UAPs' potential methods of propulsion, alongside any exotic material that could have been collected from them.

The AOIMSG was organized within Moultrie's office. Moultrie, who was sworn in last June, advises Secretary of Defense Lloyd Austin on security matters and has worked in senior roles in both the CIA and NASA. Bray, meanwhile, is an expert in intelligence collection and analysis, and works to provide intelligence summaries to the U.S. Navy.

The 2021 report, which did not draw conclusions from the "largely inconclusive" accounts, could only explain one of the sightings (which turned out to be a large, deflating balloon) and said that "we currently lack data to indicate any UAP are part of a foreign collection program or indicative of a major technological advancement by a potential adversary." It also denied that the sightings were in any way linked to clandestine tests by the U.S. military.

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The report isn't the only one the U.S. government has released that documents strange, seemingly unexplained, activity. In April, a Freedom of Information Act (FOIA) request made by the tabloid the US Sun brought more than 1,500 pages of UFO-related documents to public scrutiny, Live Science previously reported. The document database, created by the Defense Intelligence Agency's secretive Advanced Aerospace Threat Identification Program (AATIP) that ran from 2007 to 2012, included reports on more than 300 medical accounts of human interaction with UFOs — some of which included burn injuries, brain damage, nerve damage, heart palpitations and headaches from alleged close encounters with "anomalous vehicles."

Tomorrow's hearing "will give the public an opportunity to hear directly from subject matter experts, and leaders in the intelligence community, on one of the greatest mysteries of our time," said Rep. Adam Schiff, chairman of the House intelligence committee. He added that the hearing would "break the cycle of excessive secrecy and speculation with truth and transparency."

Not all representatives, however, were convinced. Arkansas Rep. Rick Crawford (R), the most senior Republican on the House intelligence subcommittee, called into question the focus on UFOs above other national security issues.

"With China and Russia developing hypersonic weapons and the Biden administration leaking alleged US military operations in Ukraine, we have far more serious intelligence threats than flying saucers," Crawford said in a statement.

Originally published on Live Science.