A top Pentagon official has attacked this week's widely watched congressional hearing on UFOs, calling the claims "insulting" to employees who are investigating sightings and accusing a key witness of not cooperating with the official U.S. government investigation.

Dr. Sean Kirkpatrick's letter, published on his personal LinkedIn page and circulated Friday across social media, criticizes much of the testimony from a retired Air Force intelligence officer that energized believers in extraterrestrial life and produced headlines around the world.

Retired Air Force Maj. David Grusch testified Wednesday that the U.S. has concealed what he called a "multi-decade" program to collect and reverse-engineer "UAPs," or unidentified aerial phenomena, the official government term for UFOs.

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Part of what the U.S. has recovered, Grusch testified, were non-human "biologics," which he said he had not seen but had learned about from "people with direct knowledge of the program."

A career intelligence officer, Kirkpatrick was named a year ago to lead the Pentagon's All-domain Anomaly Resolution Office, or AARO, which was intended to centralize investigations into UAPs. The Pentagon and U.S. intelligence agencies have been pushed by Congress in recent years to better investigate reports of devices flying at unusual speeds or trajectories as a national security concern.

Kirkpatrick wrote the letter Thursday and the Defense Department confirmed Friday that he posted it in a personal capacity. Kirkpatrick declined to comment on the letter Friday.

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He writes in part, "I cannot let yesterday's hearing pass without sharing how insulting it was to the officers of the Department of Defense and Intelligence Community who chose to join AARO, many with not unreasonable anxieties about the career risks this would entail."

"They are truth-seekers, as am I," Kirkpatrick said. "But you certainly would not get that impression from yesterday's hearing."

In a separate statement, Pentagon spokeswoman Sue Gough denied other allegations made by Grusch and other witnesses before a House Oversight subcommittee.

The Pentagon "has no information that any individual has been harmed or killed as a result of providing information" about UFO objects, Gough said. Nor has the Pentagon discovered "any verifiable information to substantiate claims that any programs regarding the possession or reverse-engineering of extraterrestrial materials have existed in the past or exist currently."

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Kirkpatrick wrote, "AARO has yet to find any credible evidence to support the allegations of any reverse engineering program for non-human technology."

He had briefed reporters in December that the Pentagon was investigating "several hundreds" of new reports following a push to have pilots and others come forward with any sightings.

Kirkpatrick wrote in his letter that allegations of "retaliation, to include physical assault and hints of murder, are extraordinarily serious, which is why law enforcement is a critical member of the AARO team, specifically to address and take swift action should anyone come forward with such claims."

"Yet, contrary to assertions made in the hearing, the central source of those allegations has refused to speak with AARO," Kirkpatrick said. He did not explicitly name Grusch, who alleged he faced retaliation and declined to answer when a congressman asked him if anyone had been murdered to hide information about UFOs.

Messages left at a phone number and email address for Grusch were not returned Friday.