Luis Elizondo made headlines in 2017 when he resigned as a senior intelligence official running a shadowy Pentagon program investigating UFOs and publicly denounced the excessive secrecy, lack of resources and internal opposition that he said were thwarting the effort.

Elizondo’s disclosures at the time created a sensation. They were buttressed by explosive videos and testimony from Navy pilots who had encountered unexplained aerial phenomena, and led to congressional inquiries, legislation and a 2023 House hearing in which a former U.S. intelligence official testified that the federal government has retrieved crashed objects of nonhuman origin.

Now Elizondo, 52, has gone further in a new memoir. In the book, he asserted that a decades-long UFO crash retrieval program has been operating as a supersecret umbrella group made up of government officials working with defense and aerospace contractors. Over the years, he wrote, technology and biological remains of nonhuman origin have been retrieved from these crashes.

“Humanity is, in fact, not the only intelligent life in the universe and not the alpha species,” Elizondo wrote.

The book, “Imminent: Inside the Pentagon’s Hunt for UFOs,” was recently published by HarperCollins after a yearlong security review by the Pentagon. Pentagon clearance does not imply endorsement.

Elizondo was, for years, a high-ranking military intelligence officer and ran highly classified programs for both the White House and the National Security Council. In 2009, he was recruited into the Advanced Aerospace Threat Identification Program, which investigated reports of UFOs — or UAP, for “unidentified anomalous phenomena,” as they are now called.

In “Imminent,” Elizondo described his struggle within the program to investigate the phenomena, and his effort, since his resignation in 2017, to push for greater transparency on what is officially known about UAP. He also wrote about personal encounters with UAP — green orbs that he said visited his home while he worked for the Department of Defense.

In the book, he expressed alarm over the potential danger to humanity posed by the existence of technology that he said far exceeds what the United States or other countries have, or can explain.

Elizondo wrote that the craft and “the nonhuman intelligence controlling them present, at best, a very serious national security issue, and at worst, the possibility of an existential threat to humanity.”

In a foreword to the book, Christopher Mellon, a former deputy assistant secretary of defense for intelligence, wrote that, without Elizondo, “the U.S. government would still be denying the existence of UAP and failing to investigate a phenomenon that may well be the greatest discovery in human history.”

The program led by Elizondo investigated sightings, near-misses and other encounters between UAP and Navy jets. It also collected data from incidents involving military and intelligence operations, including images of extraordinary craft maneuvers that were repeatedly captured by sophisticated sensors.

Within the program, he said, he learned that vehicles demonstrating “beyond next generation technology” have been observed since the 1940s. In the early 1950s, when UFOs became a Cold War national security concern, strict secrecy was enforced. “Whoever controlled such technology could control the world,” Elizondo wrote.

Much of the information collected by this program remains classified, but two unclassified Navy videos of UAP were cleared for public release at Elizondo’s request and posted by the New York Times when it broke the news of the Pentagon’s secret UFO unit in December 2017.

In an interview, Elizondo said that he had firsthand knowledge of what he was discussing, but that his security clearances prevented him from explaining the source of his knowledge. He got Pentagon approval to publish his book partly by attributing some of the information to other sources whose comments had previously been approved.

In 2007, the Defense Intelligence Agency launched the UFO- related Advanced Aerospace Weapons System Applications Program, funded with $22 million buried in an undeclared budget secured by Harry Reid, who was then the Senate majority leader.

In 2009, Elizondo became the senior ranking officer running that program’s successor, the Advanced Aerospace Threat Identification Program, along with James Lacatski and Jay Stratton. Lacatski, a rocket scientist for the Defense Intelligence Agency, and Stratton, an intelligence official with U.S. Strategic Command, were both part of the precursor program.

Frustrated by what he described as internal opposition and a lack of resources to deal with what he felt was a serious national security threat, Elizondo resigned and decided to bring his concerns to the broader intelligence community, Congress and the public.

In the memoir, Elizondo wrote of his personal encounters with UAP, describing green- glowing orbs about the size of a basketball that invaded his home on and off for over seven years. The objects were able to pass through walls, and behaved as if they were under intelligent control, he wrote.

The orbs were also witnessed by his wife, two daughters and their neighbors, he wrote.

As for “our friends from out of town,” they do not appear to be benevolent, he wrote; perhaps they are neutral. Or they could be a threat to humanity.

“We can no longer stick our heads in the sand,” he wrote. “We know we are not alone.”