A member of the Massachusetts Air National Guard, Jack Teixeira, has been arrested in connection to the leak of classified documents about Russian efforts in Ukraine and U.S. spying on its allies. The 21-year-old’s arrest was confirmed by a U.S. law enforcement official to NBC News on Thursday.

The leaked documents, which were found online last month, potentially exposed hundreds of pages of intelligence about U.S. intelligence activities in Ukraine, secret assessments of Ukraine’s combat power, and intelligence gathering on America’s allies, including South Korea and Israel, according to NBC News.

The Defense Department is currently working with the intelligence community to understand the extent of the leak, but is limited in what it can say about the documents themselves. Pentagon Press Secretary Air Force Brig. Gen Pat Ryder declined to confirm the leaker’s identity at a press briefing on Thursday, referring reporters to the Department of Justice, which is conducting a criminal investigation.

President Joe Biden suggested that officials were close to a breakthrough in the investigation. “There’s a full-blown investigation going on as you know, with the Intelligence Committee and the Justice Department, and they’re getting close,” he told reporters in Ireland on Thursday morning.

The suspect’s identity was first reported by The New York Times and The Washington Post, which identified Teixeira as the leader of a small online gaming group where the documents were first leaked. The Washington Post’s source was a minor who was granted anonymity and was a member of this group on the platform Discord. The Post said it reviewed approximately 300 photos of classified documents that the suspect allegedly leaked, most of which the report said have not been made public.

NBC News has not yet verified the details about the gaming group and whether it was the source of where the classified documents were first shared.

The Pentagon is leading an effort to review any national security implications from the leak, and it appears that some of the documents may have been altered. The Biden administration has spent the last week contacting “relevant allies and partners” at “very high levels” about the matter, according to National Security Council spokesman John Kirby.

Teixeira’s arrest has sparked concern among officials about the security of classified information and the potential for further leaks. The Department of Justice’s ongoing criminal investigation will likely shed more light on the extent of the leak and the potential damage it may have caused to U.S. national security.