US authorities on Thursday arrested the alleged leader of a group that leaked a trove of US national defence documents, Attorney General Merrick Garland told reporters on Thursday.
Mr Garland identified the man as Jack Douglas Teixeira, a 21-year-old member of the Massachusetts Air National Guard.
He was arrested in Massachusetts after a probe into the alleged unauthorised removal, retention and transmission of classified US defence information, Mr Garland said. The Justice Department did not say what charges he was facing.
"FBI agents took Teixeira into custody earlier this afternoon without incident," he said. "This investigation is ongoing."
It is believed to be the most serious security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010.
Mr Teixeira will have an initial appearance in a US district court in Massachusetts on Friday.
He allegedly led Thug Shaker Central, an online group consisting of 20-30 people who discussed a love of guns, video games and racist memes, The New York Times first reported.
The Justice Department last week opened a criminal investigation into the leaks.
President Joe Biden played down the situation earlier on Thursday, saying: “I’m concerned that it happened but there’s nothing contemporaneous that I’m aware of that is of great consequence."
The veracity of the documents remain unclear, as some US officials said they could have been altered or used as part of a misinformation campaign.
Pentagon spokesman Brig Gen Pat Ryder said the Defence Department was “very limited in what we can say” about the documents or the leaks.
“I would say, though, that it is important to understand that we do have stringent guidelines in place for safeguarding classified and sensitive information,” he told reported.
“This was a deliberate criminal act, a violation of those guidelines.”
Where the leaks had originated also remains unclear, stumping the most senior figures in Washington.
“They were somewhere in the web and where exactly, and who had access at that point, we don’t know. We simply don’t know,” Defence Secretary Lloyd Austin said on Tuesday.
Bellingcat, an independent investigative news outlet, earlier reported that the documents may have originally appeared on fringe social platforms such as Discord, a messaging site primarily used by gamers.
Agencies contributed to this report