Here's everything you need to know about the story:
What was leaked?
Leaked documents showed intelligence about the war in Ukraine and US spying on allies, the BBC reports.
There were at least 50 but perhaps more than 100 classified documents posted on Discord that shed light on international relations between the US and other countries and made claims about Putin's health.
How did they spread?
Reports allege that the leaks were kept inside a small chatroom at first called Thug Shaker Central, which included roughly 20 to 30 members from various countries.
The Washington Post reported that the group had swapped "memes, offensive jokes and idle chitchat", watched films together and prayed.
But in early March members began posting them on other Discord servers, and then the fringe message board 4chan and on the Telegram chat app, including pro-Russia channels.
Who is Jack Teixeira?
Teixeira works for the Air National Guard, which is the reserve of the US Air Force.
His official title is Cyber Transport Systems journeyman and he holds the rank of Airman 1st Class - which is a relatively junior position.
He was listed as a member of the intelligence wing of the Massachusetts Air National Guard, based at Otis Air National Guard Base in western Cape Cod.
Roads were blocked by police officers and he was arrested at his family home in Dighton, Massachusetts and is expected to make his first court appearance in Boston on Friday.
\u201cThe American media published a video of the arrest of a suspect in the leak of secret Pentagon materials.\u201d
A defence department spokesman said the Pentagon is continuing to work to "understand the scope, scale and impact of these leaks".
In a statement, Republican congressman Mike Turner - the chairman of the House Intelligence Committee - vowed to "examine why this happened, why it went unnoticed for weeks, and how to prevent future leaks"
At a news conference, defence department spokesman Brigadier General Pat Ryder said the leak was a "deliberate criminal act".
When asked how such a young airman had access to classified defence documents, Gen Ryder said that across the US military, personnel are entrusted "with a lot of responsibility at a very early age".
"Think about a young combat platoon sergeant, and the responsibility and trust that we put into those individuals to lead troops into combat," he said.
Eddy Souza, a 22-year-old man who said he went to school with Mr Teixeira, told Reuters he was surprised his former classmate had been identified as the suspect in the leaks.
"He's a good kid, not a troublemaker, just a quiet guy," Mr Souza said. "It sounds like it was a stupid kid's mistake."
“I’m not concerned about the leak,” US President Joe Biden insisted. “I’m concerned that it happened. But there’s nothing contemporaneous that I’m aware of that’s of great consequence.”
Kremlin spokesperson Dmitry Peskov told Sky News: "Like everyone else, we don't know the degree of authenticity of these documents.
"But even if we disengage from them, long before these documents appeared, we had and still have information that many instructors, including the ones from Britain, and fighters are taking part in the military actions."
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