GEOFF BENNETT: Good evening, and welcome to the "NewsHour."

The man suspected of leaking highly classified military documents has been found.

AMNA NAWAZ: The 21-year-old Massachusetts Air National Guard member was taken into custody earlier today.

Jack Teixeira -- that's him in the red shorts there -- was arrested at his home in Southern Massachusetts by heavily armed FBI agents.

He's alleged to have posted hundreds of classified documents to an online gaming platform that detail secrets about the war in Ukraine, Russian capabilities, even secrets about U.S. allies collected by American intelligence.

Attorney General Merrick Garland announced the arrest in Washington.

MERRICK GARLAND, U.S. Attorney General: FBI agents took Teixeira into custody earlier this afternoon without incident.

He will have an initial appearance at the U.S. District Court for the District of Massachusetts.

AMNA NAWAZ: Teixeira was taken to Boston, where he will make an initial court appearance tomorrow.

Nick Schifrin is following the story and joins me here.

Nick, good to see you.

NICK SCHIFRIN: Thank you.

AMNA NAWAZ: Twenty-one-year-old Jack Teixeira, who is he?

What do we know about him?

NICK SCHIFRIN: Yes, Jack Douglas Teixeira, 21 years old, served, as you just said, in the Massachusetts Air National Guard at the Otis Air National Guard base.

That's the rural part of the state.

His rank was airman 1st class.

That is very junior.

And his job title, cyber transport systems journeyman.

Now, what does that mean?

He was responsible for protecting the computer network.

He was essentially an I.T.

specialist who was supposed to be safeguarding sensitive information from external attacks, and very entry level.

That journeyman tag, that is one level above apprentice.

Now, at the same time, he was on this small group -- or led this small group on Discord.

That is an online platform popular among gamers, where he posted hundreds of documents.

And the investigative organization Bellingcat and other journalists have discovered he posted that in order to impress his friends on this platform, many of whom were teenagers, that he had access to classified information.

One of them spoke to The Washington Post about who he was.

GAMER: He did have sort of a bossy attitude at some points, but it was more of a fatherly bossy.

He did see himself as the leader of this group, and, ultimately, he was leader of this group.

And he wanted us all to be sort of super soldiers, to some degree, informed, fit, with God, well-armed, stuff like that.

NICK SCHIFRIN: So, he initially gave all of this information to his buddies.

But then the documents were reposted to a public channel.

And that started a chain reaction for it to be spread.

Millions of people read, and real military diplomatic blowback all over the world, especially among U.S. allies.

Teixeira will be charged under the Espionage Act, which makes it a crime to remove or transmit classified national defense information.

AMNA NAWAZ: Stunning details, Nick.

But this guy's airman 1st class, very junior, as you point out.

Why and how did he have access to this kind of information?

NICK SCHIFRIN: It's extraordinary think that he had access, because he was not an intelligence analyst who would need this access.

Multiple people I spoke to today compared him to Edward Snowden, the former National Security Agency contractor who also leaked hundreds of documents of very damaging classified information, who was also a computer administrator.

Teixeira, like Snowden, had access to these documents, not because he needed them for his day job, but because they were on the computer network he was supposed to secure.

Now, despite all of that, the Defense Department spokesman Brigadier General Pat Ryder spoke earlier today right before Garland's announcement, and he claimed the classified information was in the right hands.

BRIG.

GEN. PATRICK RYDER, Pentagon Press Secretary: The important thing to understand about classified information, it's not just: I want to have access to it because I have a clearance.

It's all based on need to know.

Do you have a need to know that information?

And that typically will grant you access if you have the appropriate clearances.

NICK SCHIFRIN: But Ryder and other defense officials do admit, Amna, that they're in the process of restricting the number of people who have access to these kinds of documents.

AMNA NAWAZ: Nick, what else have you learned about how authorities eventually identified and caught him?

NICK SCHIFRIN: We don't know exactly.

We will know a lot more tomorrow.

But there's two important factors that led to this being solved relatively quickly.

One, Discord, the gaming platform that I mentioned, has said publicly it cooperated with law enforcement.

And the reason that is vital is because it likely meant that investigators had the names of those people on that small group on Discord, so they could cross-reference those names with the people who have access to this kind of information.

Two, the U.S. has extensive controls over the networks where the classified information live.

And investigators are able to track who and when they access classified documents and every time someone prints out a classified document.

And, if you recall, the original leaks were actually photographs of printouts of these documents.

Now, Snowden, as an I.T.

specialist, was able to manipulate some of the logs and tried to hide what he did.

So it's possible Teixeira did the same.

But former counterintelligence officials I talk to tell me that the U.S. has taken steps to try and mitigate that risk and make sure people's digital footprints inside these networks are very visible.

But, obviously, it'll spark a lot of questions about why he had this access and why so many people have access to this information.

AMNA NAWAZ: Finally, Nick, I know you have been reporting on the fallout from the leak for days now.

What else have we learned about those leaks?

NICK SCHIFRIN: Yes, we talked about the leaks, front-line information about Ukraine, spying on allies like Israel and South Korea.

We now know a lot more about the infighting inside the Russian military.

Russian intelligence accused the Russian Defense Ministry of obfuscating the number of casualties in Ukraine.

And the head of the Wagner group -- that's Yevgeny Prigozhin -- who fought publicly with the Defense Ministry, was called in to meet President Putin and the defense minister.

It turns out that the head of the military in Russia did cut off some of the ammunition to the Wagner Group, but then changed his mind.

AMNA NAWAZ: Fascinating details.

Nick Schifrin, thank you for your reporting.

NICK SCHIFRIN: Thank you.