CNN — A member of the Massachusetts Air National Guard is expected to be arrested soon in connection with the leaking of classified documents that have been posted online, according to a law enforcement source.

The guardsman is 21-year-old Jack Teixeira, according to the New York Times, which reported that he is the leader of an online gaming group where a trove of classified documents was posted.

The FBI had narrowed the number of people who they believe could be responsible for the leaks and have been conducting interviews in recent days, two people briefed on the matter said earlier. While there’s a large number of people who had access to the documents, investigators have been able to home in on a small number for closer scrutiny thanks to the forensic trail left by the person who posted the documents. Investigators are working on building a case for prosecution, people familiar with the matter say.

The FBI believes they are close to identifying the person behind the leak. Another source familiar with the criminal investigation said that they expect it will move along much more quickly than the Pentagon’s damage assessment.

Earlier Thursday, President Joe Biden appeared to suggest that the US government was close to identifying the leaker.

“There’s a full-blown investigation going on, as you know,” Biden said when asked for comment about the leaks. “The intelligence community and the Justice Department. And they’re getting close. I don’t have an answer for you.”

Biden was speaking in Dublin, where he is meeting the Irish president. It was the first time he commented on the leak.

CNN has previously reported that the Army Criminal Investigation Division is also “assisting the DoD in their investigation” of the leak, Jeffrey Castro, a spokesman for the division, told CNN.

On Wednesday, the Washington Post reported that the person behind the leak worked on a military base and posted sensitive national security secrets in an online group of acquaintances.

The leaker is described in the Post story as a lonely young man and gun enthusiast who was part of a chatroom of about two dozen people on Discord – a social media platform popular with video gamers – that shared a love of guns and military gear, according to a friend of the alleged leaker the Post interviewed who was also part of the group.

The friend told the Post he would not reveal the identity of the alleged leaker, who goes by the moniker “OG,” or their location to authorities.

Biden said he was concerned about the fact the leaks happened, but not necessarily about their content.

“I’m not concerned about the leak. I’m concerned that it happened, but there’s nothing contemporaneous that I’m aware of that is of any consequence.”

The leaked documents posted to social media, some of which have been obtained by CNN, include detailed intelligence assessments of allies and adversaries alike, including on the state of the war in Ukraine and the challenges both Kyiv and Moscow face as the war appears stuck in a stalemate in the months ahead.

The Pentagon has begun to limit who across the government receives its highly classified daily intelligence briefs following a major leak of classified information discovered last week.

Some US officials who used to receive the briefing materials daily have stopped receiving them in recent days, sources familiar with the matter told CNN, as the Pentagon’s Joint Staff continues to whittle down its distribution lists.

The Joint Staff, which comprises the Defense Department’s most senior uniformed leadership that advises the president, began examining its distribution lists immediately after learning of the trove of leaked classified documents – many of which had markings indicating that they had been produced by the Joint Staff’s intelligence arm, known as the J2.

Pentagon spokesman Brig. Gen. Pat Ryder in an interview with News Nation on Wednesday said the Pentagon is looking at “mitigation measures in terms of what we can do to prevent potential additional unauthorized leaks.”

The criminal investigation is being led by the FBI’s Washington field office, including a team of counterintelligence investigators experienced in hunting leaks.

Those investigators are also working with Pentagon officials on the damage assessment, which would become part of the evidence to be used in any potential prosecution that results.

CNN’s Zachary Cohen, Sean Lyngaas and Alex Marquardt contributed reporting.