The FBI arrested a 21-year-old member of the Massachusetts Air National Guard as part of an investigation into the leak of highly classified US intelligence documents.

Attorney-general Merrick Garland identified the suspect as Jack Teixeira in a brief statement to reporters on Thursday. “FBI agents took Teixeira into custody earlier this afternoon without incident,” Garland said, adding that he would appear in federal court in Massachusetts.

More than 100 classified documents have been leaked on social media, starting on the messaging platform Discord, some of which were published on Telegram and Twitter last week. They largely focus on the war in Ukraine but also include information intercepted by the US from allies including South Korea and Israel.

President Joe Biden had said earlier on Thursday that there was “a full-blown investigation going on . . . with the intelligence community and the justice department”.

A Pentagon spokesperson, Brigadier General Pat Ryder, on Thursday described the leak as “a deliberate criminal act.”

US officials said they were starting to assess the fallout from the leak even as they try to understand its full extent.

The Pentagon is also conducting its own review on who has access to sensitive information and how it is distributed. The defence department has already begun clamping down on who receives daily intelligence briefings and as well as its most sensitive information, US officials said Thursday.

Biden tried to play down the consequences of the leak. While the documents do not contain battle plans for Ukraine’s coming counteroffensive against Russia, they offer a more downbeat picture of the state of Ukrainian forces and their ability to make significant gains on the battlefield in the coming months.

“I’m concerned that it happened, but there’s nothing contemporaneous that I’m aware of that is of great consequence right now,” the president said.