“We don’t know,” Rubio added. “But how can we have stuff flying over restricted military airspace and not even be curious – not to mention concerned – about who it is and why they’re here?”

The Pentagon took a significant step forward on the issue last year, establishing an Unidentified Aerial Phenomena Task Force. It has since urged military pilots to report unusual sightings, some of which have been detected to be flying at hypersonic speeds.

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The report set for release this week, which Congress mandated, is intended to end the stigma often associated with UAPs and emphasise to Americans that the intelligence community views the sightings as a major vulnerability for the US.

Republicans and Democrats alike viewed those steps as positive but long overdue, especially since classification barred lawmakers from publicly discussing the nature of the threats. While lawmakers stayed mum, some former US presidents openly teased juicier details – in the case of some airborne objects, “we can’t explain how they moved,” Barack Obama told late-night television host Stephen Colbert last month.

“There’s a lot of interest on the committee,” said Senator Mark Warner, who chairs the Intelligence panel and has learned of sightings off the coast of his home state. “When you have this much visual and radar evidence that there is something … we’ve got to get an answer.”

As keen as they are to get more information, members of Congress who have followed the unidentified-object issue closely over the years said in interviews that they do not expect earth-shattering revelations in the unclassified version of the forthcoming report from the Office of the Director of National Intelligence and the Pentagon. That report is likely to detail the process by which US officials are seeking to identify and learn more about the unexplained aircraft.

Rubio, whose perch on the Intelligence Committee puts him in a selective group of eight lawmakers privy to the nation's most sensitive intelligence, said he has “no reason to believe” that the report will give credence to theories about extraterrestrials. Instead, he said, the report will detail “advanced technologies that we don’t possess [that] are in our restricted airspace almost on a nightly basis now.”

“How do we not take that seriously?” he added.

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Congressional officials on both sides of the Capitol have been briefed in the run-up to the highly-anticipated release of the report. But Warner and Rubio said they hadn’t yet been briefed on it; Warner said he has asked the Director of National Intelligence Avril Haines for a “pre-briefing”.

Last week, though, members of the House Intelligence Committee and the House Armed Services Committee were briefed by Brennan McKernan, the director of the Pentagon’s UAP task force, and Scott Bray, the deputy director of naval intelligence.

Sources familiar with the briefings said the officials walked through the steps the Pentagon is taking to try to identify the unexplained sightings and figure out their source or sources. Pentagon officials also used the briefings to highlight the potentially advanced nature of the technology being used.

“There have been questions about the sightings and whether they’re natural phenomena, whether they’re drones, whether they’re weather balloons or vehicles from nation-states or non-state actors spying on our military or spying on US citizens,” said congressman André Carson, who chairs a key subpanel on the Intelligence Committee.

“It’s always concerning when some of our foreign adversaries have a competitive technological edge over us,” added Carson, who participated in the briefings.

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The report is also expected to disprove some of the more wacky theories about certain sightings, which have fuelled conspiracy theories about extraterrestrial life. The fact that the US government is even producing a report on the matter – sought by Congress in last year’s defence policy bill – shows that top officials are trying to move past the sci-fi reputation of UAPs.

“There are so many un-credible people in that subject matter,” Senator Jim Inhofe lamented in a brief interview.

Inhofe, the top Republican on the Senate Armed Services Committee, said he was looking forward to the release of the report, which he said was critical “for no other reason than to discredit a lot of the things that shouldn’t be credited.”

“See, I’m a lot older than anybody else here … and yet I remember, when I was a little kid – it goes way, way back,” Inhofe, 86, said of still-brewing theories about extraterrestrial life.

Olivia Beavers contributed to this report.

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