For his high school senior yearbook, Jack Teixeira got to pick a quote to appear below the photo showing him smiling in a green hoodie. His choice: “Actions speak louder than words.”

On Friday, Mr. Teixeira, the 21-year-old airman in the Massachusetts Air National Guard, was charged with two counts under the Espionage Act because, federal prosecutors say, his actions caused harm to the country he had devoted himself to serving. He did not enter a plea.

They said he repeatedly shared classified documents on the internet through a gaming group he belonged to that focused on war, guns, and sometimes, racist and antisemitic memes. It was a remarkable turnabout for a young man who grew up with a passion for the military and weapons — at times to an unnerving extent, some who knew him said.

Airman Teixeira grew up in a family with strong military ties in Dighton, Mass., a town of about 8,000 people near the Massachusetts-Rhode Island border that retains a distinctly rural character, with tractors in backyards, fresh eggs for sale in roadside honor boxes and old stone walls rambling through woods. It is also reliably conservative.

The Teixeira home sits at the end of a long, densely treed lot, where Airman Teixeira’s mother runs a home floral business. His stepfather retired as a master sergeant from the same Air Force intelligence unit where Airman Teixeira worked, headquartered on a base on nearby Cape Cod. He also has a stepbrother in the Air Force, according to a profile for the brother on LinkedIn.

Close to two dozen people who knew Airman Teixeira when he was in high school did not respond to interview requests or declined to comment. But several former classmates at Dighton-Rehoboth Regional High School remembered him as a quiet and somewhat awkward teenager who sometimes wore camouflage and boots to school.

Kailani Reis, 20, a high school classmate in Airman Teixeira’s graduating class, said that as a student, the airman expressed his interest in weapons often enough that she and some other students found it “unsettling” and avoided him. She said few of the former classmates she knew were surprised when he was arrested.

She did not recall that he talked politics but said he spoke often about joining the military and on occasion told teachers who encouraged him that he did not plan on going to college.

He even skipped his high school graduation in 2020 to report to basic training in the Air Force at Lackland Air Force Base in Texas.

“Congratulations to my son Jack Teixeira who was not able to attend the ceremony,” his mother posted on Facebook at the time. “It’s too bad they didn’t mention his name at some point.”

In the Air Force, Airman Teixeira became a low-level computer tech at Otis Air National Guard Base in Sandwich, Mass., where his mother said he worked nights, helping maintain secure networks. There, he had broad access to a secure facility where he could access a global network of classified material from the military and 17 other American intelligence agencies.

Authorities say that Mr. Teixeira eventually leaked dozens of documents containing potentially harmful details about the war in Ukraine and other sensitive national security topics.

That a 21-year-old with so little authority could have access to a such a vast trove of top secret information might surprise the general public, but people who have worked in the intelligence world say untold thousands of troops and government civilians have access to top secret materials, including many young, inexperienced workers the military relies on to process the monumental amount of intelligence it collects.

Those workers can log onto the Joint Worldwide Intelligence Communications System — essentially a highly classified version of Google — and in milliseconds pull up briefings on Ukraine, China or nearly any other sensitive subject that the U.S. government collects intelligence on.

Though his motivations may be different, Mr. Teixeira is remarkably similar to two other high-profile leakers in recent years, Chelsea Manning and Reality Winner, said Javed Ali, a former senior U.S. counterterrorism official who held intelligence roles at the F.B.I., the Defense Intelligence Agency and the Department of Homeland Security.

Ms. Manning was a 23-year-old Army intelligence analyst who was convicted in 2013 of giving more than 700,000 classified documents to WikiLeaks. Ms. Winner was a 26-year-old former Air Force linguist working as a military contractor who in 2017 printed out a classified report on Russian hacking, hid it in her pantyhose, and gave it to The Intercept.

Unlike Ms. Manning and Ms. Winner, who came to be seen as whistle-blowers motivated by ideology, Airman Teixeira did not appear to be driven by government policies, according to people who knew him online.

But all three were relatively young and had security clearances that were the classified intelligence equivalent of having the keys to dad’s red convertible.

“Clearly their relatively young age is a common factor, and I would hope the intelligence community is thinking about that,” said Bennett Miller, a retired Air Force intelligence analyst. “The problem is that the community needs these people. It can’t work without them.”

The words “top secret” may conjure images of pristine vaults and retinal scanners, Mr. Miller said, but in reality, while some highly classified material is siloed in special access programs, most of the rest is accessible to thousands of ordinary people who have security clearances. And security can be surprisingly lax.

Often, these systems are basically just a bunch of computers on a desk and there is “nothing really stopping anyone from printing something and carrying it out,” Mr. Miller said, adding, “It ain’t as Gucci as people think.”

According to some of the gamers who belonged to a small group created by Airman Teixeira on the social media site Discord, he liked to play the apocalyptic zombie game Project Zomboid, as well as Arma 3, a tactical shooter game known for its lifelike attention to detail. He also liked to lecture the Discord group about the war in Ukraine and conflicts around the globe, sometimes typing for a half-hour at a time to share what he was learning from classified intelligence at work.

One group member told The New York Times that the briefings were largely ignored by the group, so Airman Teixeira, in frustration, started posting photos of actual classified materials. Those materials were later shared on other online platforms, eventually attracting the attention of federal authorities, who arrested him at gunpoint at his home on Thursday.

Restricting access to classified material might lead to fewer leaks, Mr. Miller said, but it also might cut off the flow of information, leading to intelligence failures that are far more dangerous than what any of the leakers have done.

“That’s what happened before 9/11,” he said. “We were too locked down and couldn’t put together the pieces of the puzzle. As we look for a fix, we have to make sure we don’t make the same mistake.”

At a Pentagon briefing on Thursday, Brig. Gen. Patrick Ryder, a spokesman, addressed concerns about Airman Teixeira’s age, noting that the military routinely gives young people tremendous responsibility.

“Think about a young combat, you know, platoon sergeant, and the responsibility and trust that we put into those individuals to lead troops into combat,” he said. “You receive training and you will receive an understanding of the rules and requirements that come along with those responsibilities, and you’re expected to abide by those rules, regulations and responsibility. It’s called military discipline. And in certain cases, especially when it comes to sensitive information, it also is about the law.”

Sheelagh McNeill and Susan C. Beachy contributed research. Eric Schmitt contributed reporting.