David Grusch testified that he "absolutely" believes the government is in possession of unidentified anomalous phenomena, or UAPs -- which has replaced the term UFO in official parlance -- as well as remains of their non-human operators.

"I was informed, in the course of my official duties, of a multi-decade UAP crash retrieval and reverse-engineering program," Grusch said.

"I made the decision, based on the data I collected, to report this information to my superiors and multiple inspectors general, and in effect becoming a whistleblower," he said.

Pressed for details in the course of the hearing, Grusch repeatedly said he could not comment in a public setting because the information is classified.

He said the US government is hiding information on UAPs not only from the public but from Congress, and that he personally interviewed people with direct knowledge of non-human craft.

"My testimony is based on information I've been given by individuals with a longstanding track record of legitimacy and service to this country -- many of whom also shared compelling evidence in the form of photography, official documentation and classified oral testimony," Grusch told lawmakers.

US Representative Tim Burchett supported the idea that the government was concealing information, saying at the opening of the hearing -- which also featured testimony from two former Navy officers who said they witnessed UAPs -- that "we're going to uncover the cover-up."

- 'We don't have the answers' -

"This is an issue of government transparency. We can't trust a government that does not trust its people," he said.

Asked if there may be life beyond Earth, National Security Council spokesman John Kirby said he does not have a position on the issue one way or another.

"What we believe is that there are unexplained aerial phenomena that have been cited and reported by pilots -- Navy and Air Force," he said, adding: "We don't have the answers about what these phenomena are."

The head of the Pentagon office set up to identify UAPs that pose a potential threat also told lawmakers earlier this year that it had not identified signs of alien activity.

The All-domain Anomaly Resolution Office "has found no credible evidence thus far of extraterrestrial activity, off-world technology, or objects that defy the known laws of physics," Sean Kirkpatrick testified in April.

The US government has however begun taking the issue of UAPs more seriously in recent years.

NASA held its first public meeting on UAPs in May, calling for a more rigorous scientific approach to clarify the origin of hundreds of mysterious sightings.

The Pentagon also began paying closer attention to the subject after a slew of inexplicable sightings from US Navy and Air Force pilots.

The central worry was that the sightings could be of unknown aerial surveillance technology used by China to collect intelligence on US defenses.

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