The person suspected of recently leaking classified U.S. government documents has been identified as Jack Teixeira, a 21-year-old member of the Massachusetts Air National Guard, two law enforcement officials said Thursday.

Officials, who spoke on the condition of anonymity to discuss an ongoing investigation, said they have been onto him for some time and that an arrest is imminent.

The classified documents from the Department of Defense were found online last month — it remains unclear how long the documents had been on the internet and the total number that have been posted — and revealed details of U.S. spying on Russia’s war efforts in Ukraine, secret assessments of Ukraine’s combat power, as well as intelligence gathering on America’s allies, including South Korea and Israel, NBC News previously reported.

President Joe Biden suggested on Thursday morning that officials appeared to be nearing a breakthrough in their investigation into who leaked the documents online.

“There’s a full-blown investigation going on as you know, with the Intelligence Committee and the Justice Department, and they’re getting close,” Biden told reporters in Ireland.

The suspect’s identity was first reported on Thursday by The New York Times, which said he was the leader of a small online gaming group where he first leaked the classified documents.

The Washington Post was the first news outlet on Wednesday night to report about the gaming group, and only identified him as “OG.” The main source of the story 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 that it was the source of where the classified documents were first shared.

Biden and Defense Secretary Lloyd Austin were briefed about the disclosure last week, administration officials said. That was when the White House first learned about the existence of the documents in the public domain, National Security Council spokesman John Kirby told reporters earlier this week.

According to Bellingcat, the online open-source investigative group, the documents appeared in early March on the Discord social media app. Some documents may have appeared as early as January, the group said.

Kirby said the Pentagon is “leading an interagency effort here to review whatever national security implications might come out of all this” and said the Department of Justice is leading a criminal investigation into the leaks.

He also said that it appears that some of the classified documents had been altered from their original form. Kirby said that Biden administration officials spent last weekend contacting “relevant allies and partners” and “at very high levels” to reassure them about the leaks.