GEOFF BENNETT: Are we alone in the universe?

One former Pentagon employee told Congress last week that the truth is out there and it's being hidden from the American people.

Former intelligence officer David Grusch says he worked on classified military programs and that UFOs are real.

And the government, he says, has been withholding knowledge of nonhuman life on earth for nearly a century.

Meantime, NASA is set to release its own report on UFOs in the next few weeks.

Here to help us separate fact from science fiction is journalist Garrett Graff, whose forthcoming book is "UFO: The Inside Story of the U.S. Government's Search for Alien Life Here - and Out There."

Thanks for being with us, Garrett.

GARRETT GRAFF, Author, "UFO: The Inside Story of the U.S. Government's Search for Alien Life Here - and Out There": Always a pleasure.

GEOFF BENNETT: One of the reasons why this congressional hearing resonated with so many people and one of the reasons why we're still talking about it is because that former Air Force intelligence officer told Congress that the U.S. government has a longstanding program that retrieves UFOs.

And he also said that -- quote -- "Nonhuman biologics have been found at crash sites."

DAVID GRUSCH, Former U.S. Intelligence Officer: As I have stated publicly already in my NewsNation interview, biologics came with some of these recoveries, yes.

REP. NANCY MACE (R-SC): Were they, I guess, human or nonhuman biologics?

DAVID GRUSCH: Nonhuman.

And that was the assessment of people with direct knowledge on the program I talked to that are currently still on the program.

GEOFF BENNETT: So, based on your own reporting and research, what should we make of that assertion?

And does nonhuman mean alien?

GARRETT GRAFF: I think, in this case, that's what David Grusch intends it to mean.

I think the challenge is, we don't have anyone saying that they have firsthand knowledge of these finds.

In that sense, David Grusch actually fits into a long tradition dating back into the 1970s and 1980s of these sorts of so-called UFO whistle-blowers who come forward with what the UFO community calls FOAF tales, not folk tales, but FOAF tales, friend of a friend tales who say sort of: This is what I have heard.

I don't have firsthand knowledge.

GEOFF BENNETT: Well, to your point.

I mean, for decades, the discussion around UFOs had been sort of characterized by stereotypes of people in tinfoil hats.

But, here, you have veteran pilots, former intelligence officials, people who, by all appearances, are credible.

Is what they're saying also credible and reliable?

GARRETT GRAFF: Yes, and I think this is part of a very historic change that we have seen since 2017, when there were new revelations around the Pentagon's engagement on what used to be called UFOs that the government now calls UAPs, unidentified anomalous phenomenon.

And you now have serious people saying, as these two Navy fighter pilots, naval aviators, said in their hearing last week, there are technologies out there that we are encountering that we cannot explain and that are better than anything that we think our government possesses.

GEOFF BENNETT: When did the U.S. government start tracking UFOs, or UAPs?

And help us understand how this discussion has sort of fallen into public view, because, for a long time, lawmakers and officials weren't willing to talk about this.

GARRETT GRAFF: Yes, the modern era of UFOs really started right after World War II; 1947 was the beginning of what you might call the flying saucer era, in terms of people spotting things in the sky that they thought were extraterrestrial.

And from the 1940s until the late 1960s, early 1970s, the U.S. government had a very active UFO sort of search and study program.

But that was shut down.

And for about 50 years, the Pentagon disavowed any knowledge that it had a meaningful UFO program, until around 2017, when you began to see this modern resurgence and this sort of new era where people are grappling with the reality, the demonstrated, clear reality, that there are technologies out there that our military is encountering that they cannot explain.

GEOFF BENNETT: Well, I was going to ask you how most UFO encounters are explained.

But I hear you say that there's a good portion of them that aren't.

GARRETT GRAFF: Yes.

And some of the government's renaming of this from UFOs to UAPs, unidentified aerial phenomenon, anomalous phenomenon, is to try to capture the idea that some of this is -- are probably not objects, that some of this, it might be atmospheric, meteorological, astronomical phenomenon that we don't understand.

But some chunk of it is certainly advanced aerial technologies being tested by adversaries like China, Russia, and Iran.

One of the things that the Pentagon has said that its UAP program has discovered is a heretofore unknown Chinese transmedium drone.

That is a Chinese craft that comes out of the water and transitions to flight.

GEOFF BENNETT: The U.S. government has long been accused of hiding what it knows about UFOs, or UAPs.

In fact, at that congressional hearing, there was one Republican member who said that the government's lack of transparency surrounding UAPs amounts to a cover-up.

That was the phrase that he used.

Based on your reporting, is there evidence of a coordinated government cover-up?

GARRETT GRAFF: Well, the government certainly covers up some aspects of this, what its sensors can pick up.

Some chunk of this is surely our own advanced technologies being tested, stealth craft, et cetera.

But I think, actually -- and this is what I sort of tried to trace in the book -- is, over the last 75 years, a lot of this has been, I think, a cover-up of ignorance, not of knowledge, which is to say that this is the fact that the government is uncomfortable saying there's stuff out there and we don't know what it all is.

GEOFF BENNETT: As we wrap up our conversation, I want to try to answer the question we posited in the opening.

So far as we know, only Earth can sustain and host life.

Is there life on other planets?

GARRETT GRAFF: The last 25 years of science, the last decade of science leads to the almost inescapable conclusion that there is probably not just life out there, but probably intelligent life.

The math is really on the side of extraterrestrial civilizations out there.

The challenge is, is, any of it close enough that we will ever notice it?

GEOFF BENNETT: Garrett Graff, looking forward to reading your forthcoming book on this topic.

Thanks so much for being with us.

GARRETT GRAFF: Always a pleasure.