The FBI wants to question a 21-year-old member of the Massachusetts Air National Guard in connection with the disclosure of classified military documents, two people familiar with the case said Thursday.

The two people who discussed the development were not authorized to discuss an ongoing investigation publicly and spoke to The Associated Press on condition of anonymity.

The New York Times previously reported on the FBI’s interest in the person.

The Justice Department and FBI have narrowed the pool of potential suspects believed responsible for the disclosure of top-secret military documents about Ukraine, a person familiar with the investigation said Thursday.

The Biden administration has been working to assess the diplomatic and national security implications of the leaked documents since they were first reported last week. A top Pentagon spokesman told reporters earlier this week that the revelations pose a very serious risk to national security, and the Justice Department opened an investigation to identify the person responsible.

We are getting close, President Joe Biden told reporters in Ireland on Thursday. He said that while he was concerned that sensitive government documents had been released, “there is nothing contemporary that I know is of major importance.

Since the investigation began, officials have been trying to figure out the number of suspects who may be responsible, according to a person with knowledge of the investigation who was not authorized to discuss an ongoing investigation. It was not clear how many suspects were left and whether or when an arrest could take place.

The Justice Department declined to comment on Thursday.

It’s possible that the leak started on a site called Discord, a social media platform popular with people who play online games. The Discord site hosts real-time voice, video, and text chats for groups and describes itself as a place where you can be a member of a school club, gaming group, or global arts community.

In one of those forums, originally created to discuss a range of topics, members debated the war in Ukraine. According to a member of the chat, an unidentified poster shared documents that the poster claimed were classified, first typed them out with the poster’s own thoughts, and then, from a few months ago, uploaded images of folded papers.

Discord has said it is cooperating with the police.

There are only a few ways the leaked classified information has been accessed, which could provide critical clues as to who is responsible. Typically in classified briefings, such as the slides posted on Discord, the information is shared electronically.

This is done via secure computer terminals where users gain access based on their login details or via tablets that are handed out for briefings and collected later. If the slides need to be printed, they can only be sent to secure printers that can handle confidential documents and keep a digital record of anyone who has requested a print.

It’s those digital clues, like the record of prints, that can help investigators figure out who took the documents. In most photos of documents posted online, the photos of paper copies that look like they’ve been folded in four are almost like they’ve been put in someone’s pocket.

In the days since the leaks came to light, the Pentagon deferred questions about the investigation to the Justice Department, saying it is a criminal matter. Even if the person who leaked the files is an active member of the U.S. military, the Justice Department would likely still be in charge of the prosecution until it’s ready to turn the case over to the Department of Defense, a spokesman said. Defense official to the AP on condition of anonymity to discuss sensitive matters.

This was the case in previous criminal cases, such as when two Marines were arrested at Camp Pendleton in California in 2020 on charges of drug trafficking, the official said. If the person responsible turns out to be a civilian, the Justice Department will be responsible, the official said.

CNN reported earlier Thursday that the FBI thought it was getting close to identifying a suspect.

Associated Press writers Colleen Long and Darlene Superville in Dublin contributed to this report.