NORTH DIGHTON, Mass. — The F.B.I. arrested a 21-year-old member of the Massachusetts Air National Guard on Thursday in connection with the leak of dozens of highly classified documents containing an array of national security secrets, including the breadth of surveillance the United States is able to conduct on Russia.
Airman First Class Jack Douglas Teixeira was taken into custody to face charges of leaking classified documents after federal authorities said he had posted batches of sensitive intelligence to an online gaming chat group, called Thug Shaker Central.
As reporters from The New York Times gathered near the house on Thursday afternoon, about a half-dozen F.B.I. agents pushed into the home of Airman Teixeira’s mother in North Dighton, with a twin-engine government surveillance plane keeping watch overhead.
Some of the agents arrived heavily armed. Law enforcement officials learned before the search that Airman Teixeira was in possession of multiple weapons, according to a person familiar with the investigation, and the F.B.I. found guns at the house.
Not long after, cameras caught a handcuffed Airman Teixeira, wearing red shorts and boots, being led away from the home by two heavily armed men.
In Washington, Attorney General Merrick B. Garland, in a brief statement, announced the arrest and said Airman Teixeira would be arraigned at the Federal District Court in Massachusetts. Mr. Garland said he was arrested in connection with the “unauthorized removal, retention and transmission of classified national defense information,” a reference to the Espionage Act, which is used to prosecute the mishandling and theft of sensitive intelligence.
The arrest raised questions about why such a junior enlisted airman had access to such an array of potentially damaging secrets, why adequate safeguards had not been put in place after earlier leaks and why a young man would risk his freedom to share intelligence about the war in Ukraine with a group of friends he knew from a video game social media site.
A motive in the case for now remains elusive. But, according to people who knew him online, Airman Teixeira was no whistle-blower. Unlike previous huge leaks of information, from the Pentagon Papers to WikiLeaks to Edward Snowden’s disclosures, outrage about wrongdoing or government policies does not appear to have been a factor.
Indeed, the disclosures were potentially damaging to all parties in the Ukraine war as well as future intelligence collection. While some officials, including President Biden, have downplayed the damage from the leak, it will take months to learn whether U.S. intelligence loses access to important methods of collection because of the disclosures.
The F.B.I. had been zeroing in on Airman Teixeira for several days, tracking its own investigative clues as well as some of the same information that The Times and The Washington Post had developed about the Discord group where he had shared the documents, officials said.
Still, as reporters uncovered more information, law enforcement officials had to speed up their investigation.
While federal investigators believed that Airman Teixeira could pose a danger to agents conducting the search, his online friends knew him as a sometimes hectoring leader of their small community.
Several months ago, a user of Thug Shaker Central known as O.G. began uploading hundreds of pages of intelligence briefings into the small chat group. The group also discussed guns and military equipment, as well as the original subject of their group, video games.
While the members of the chat group would not identify the group’s leader by name, a trail of digital evidence compiled by The Times led to Airman Teixeira. American officials have confirmed that they believe he uploaded the information taken improperly from U.S. military computers.
As he posted the material, O.G. lectured the group’s members, who had bonded during the isolation of the pandemic, on the importance of staying abreast of world events.
Airman Teixeira was trained as a cyber transport systems specialist, a job that could entail a variety of duties, such as keeping his unit’s communication networks running. He was assigned to the 102nd Intelligence Wing at Otis Air National Guard Base, part of Joint Base Cape Cod, according to an Air Force spokeswoman. The 102nd Intelligence Wing’s official Facebook page congratulated Airman Teixeira and colleagues on their promotion to airmen first class in July.
Officials would not answer questions about what in Airman Teixeira’s duties would necessitate his having access to daily slides about the Ukraine war, much less the daily deluge of intelligence reports from the C.I.A., the National Security Agency and the Office of the Director of National Intelligence. There are units at the base that process intelligence collected from drones and U-2 spy planes, though it is doubtful that work alone would require the sort of access to the broad array of classified information that has been leaked on the Discord server.
But he could also have gained access to the documents in other ways. U.S. government officials with security clearance often receive such documents through daily emails on a classified computer network, one official told The Times, and those emails might then be automatically forwarded to other people.
Airman Teixeira’s mother, Dawn, speaking outside her home in North Dighton on Thursday, confirmed that her son was a member of the Air National Guard and said he had recently been working overnight shifts at a base on Cape Cod. In the past few days, he had changed his phone number, she said.
Later, someone who appeared to be Airman Teixeira drove onto the property in a red pickup truck.
When Times reporters approached the house again, the truck was parked in the driveway. Airman Teixeira’s mother and his stepfather were standing in the driveway.
When asked if Airman Teixeira was there and willing to speak, his stepfather, Thomas P. Dufault, said: “He needs to get an attorney if things are flowing the way they are going right now. The feds will be around soon, I’m sure.”
Within a few hours, the prediction of Mr. Dufault, a retired Air Force master sergeant, proved correct as F.B.I. and other government personnel drove onto the property.
A neighbor, Paul Desousa, looked on as the F.B.I. agents yelled Airman Teixeira’s name. The neighbor said the young man then walked out of the house.
Mr. Desousa did not know Airman Teixeira, but said he had frequently heard him fire weapons in the woods behind his house.
After Airman Teixeira was led away, the search of the property continued. And as the sun began to set, a food delivery truck arrived for the F.B.I. agents scouring Airman Teixeira’s family home, a sign that the search was likely to continue for several hours.
Members of Thug Shaker Central who spoke to The Times said that the documents they discussed online were meant to be purely informative. While many pertained to the war in Ukraine, the members said they took no side in the conflict.
The documents, they said, started to get wider attention only when one of the teenage members of the group took a few dozen of them and posted them to a public online forum. From there they were picked up by Russian-language Telegram channels and then The Times, which first reported on them.
In Washington, the crisis over the leaks began late last week, as some documents began surfacing on Telegram and Twitter.
Defense Secretary Lloyd J. Austin III was initially briefed on the leak on the morning of April 6. Pentagon officials tried to get some of the Telegram and Twitter posts showing pictures of some of the documents that initially came to light deleted, but they were unsuccessful.
The next day, last Friday, Mr. Austin began convening departmentwide meetings to address the growing disclosures. Pentagon and other U.S. officials began contacting congressional leaders and allies to alert them to the leaks, which have ignited political firestorms in some countries.
Also last Friday, the military’s Joint Staff, which had produced many of the briefing slides that were leaked, instituted procedures to limit the distribution of highly sensitive briefing documents and restrict attendance at meetings where briefing books containing paper copies of the documents were available.
On Tuesday, in his first public comments about the leaks, Mr. Austin struggled to explain why the Defense Department only learned about the leaks long after they first surfaced on Discord.
“Well, they were somewhere in the web,” Mr. Austin said of the leaked documents. “And where exactly and who had access at that point, we don’t know. We simply don’t know at this point.”
Even as Mr. Austin spoke, news outlets began writing about discoveries of more documents.
On Thursday morning, Mr. Austin called a meeting with senior staff members to discuss the crisis.
But by then the F.B.I. was already preparing the search warrant for the home in North Dighton, and investigators began assuring Pentagon officials that the leaker would soon be caught.
Reporting and research were contributed by Riley Mellen, Adam Goldman, Michael Schwirtz, Helene Cooper, Eric Schmitt, John Ismay, C.J. Chivers, Michael D. Shear, Kitty Bennett and Sheelagh McNeill.