Teixeira was arrested “in connection with the unauthorized removal, retention and transmission of classified national defense information,” the attorney general said, using language that tracks violations of the Espionage Act.

No specific charges were immediately announced, but Teixeira is expected to appear in federal court in Boston on Friday.

During a hastily-assembled appearance before reporters at Justice Department headquarters, Garland spoke for less than a minute and provided no other details about the investigation beyond saying that it was “ongoing.”

An FBI statement also confirmed Teixeira’s arrest and said it related to “his alleged involvement in leaking classified U.S. government and military documents.”

“Since late last week the FBI has aggressively pursued investigative leads and today’s arrest exemplifies our continued commitment to identifying, pursuing, and holding accountable those who betray our country’s trust and put our national security at risk,” the FBI statement added.

The 21-year-old appears to have been part of a small group on the Discord social media platform. He first wrote about the sensitive information in written paragraphs, paraphrased from the documents, months ago, as POLITICO previously reported. Starting in January, he began posting photographs of printouts of the documents, which had been folded and then smoothed out.

The New York Times was the first to report that the likely leaker was Teixeira and said he was a member of the intelligence unit of the Massachusetts Air National Guard.

The Washington Post first reported Thursday that the individual who leaked the documents on Discord worked on a military base. Teixeira was reportedly considered the leader of the small Discord channel, the Post reported, and espoused a love for guns and God.

The documents Teixeira allegedly leaked contained highly classified information, including from papers marked “Top Secret,” about the war in Ukraine and other global topics such as China, Iran and the Russian paramilitary group, Wagner.

President Joe Biden, speaking to journalists earlier in the day during a trip to Ireland, seemed to downplay the gravity of the breach which has roiled the intelligence community, the Pentagon and U.S. relationships with a variety of allies.

“I’m concerned that it happened. But there’s nothing contemporaneous that I’m aware of that’s of great consequence,” Biden said while outside the residence of his Irish counterpart.

But the public leak of classified intelligence is the largest since Wikileaks, which from 2006 to 2021 led to the publication of millions of emails, documents and other sensitive materials online.

While the recent breach is much smaller in scale, the documents exposed in extraordinary detail the extent to which the U.S. spies on its allies and adversaries and included analyses that had been compiled just weeks before they were posted. The papers exposed battlefield planning by both the Ukrainians and the Russians, including detailed maps of troop movements, and that the U.S. had asked South Korea to provide Kyiv with ammunition.

The Biden administration first began looking into the leak last week, including how the documents first ended up online and how they were able to circulate for months without detection. The Justice Department is leading the interagency investigation.

DoD is reviewing its policies related to safeguarding classified material, including assessing how and where intelligence is shared, Pentagon spokesperson Brig. Gen. Patrick Ryder said Thursday.

“It’s important to understand that we do have stringent guidelines in place ... this was a deliberate, criminal act, a violation of those guidelines,” Ryder said. “Anyone who violates those rules is doing so willfully.”

Lara Seligman contributed reporting.