WASHINGTON, D.C. – On Thursday, the FBI arrested Jack Douglas Teixeira, a 21-year-old member of the U.S. Air National Guard, over the leaks online of classified documents that embarrassed Washington with allies around the world.

Attorney General Merrick Garland said the Federal Bureau of Investigation arrested Teixeira “in connection with an investigation into alleged unauthorized removal, retention, and transmission of classified national defense information.”

“FBI agents took Teixeira into custody earlier this afternoon without incident,” Garland told reporters in a brief statement at the Justice Department.

“This investigation is ongoing,” Garland said. He did not take questions.

The FBI said its agents had made an arrest and were conducting “authorized law enforcement activity at a residence in North Dighton, Massachusetts.”

“Since late last week the FBI has aggressively pursued investigative leads and today’s arrest exemplifies our continued commitment to identifying, pursuing and holding accountable those who betray our country’s trust and put our national security at risk,” a bureau statement read.

Video played on news channels showed heavily armed officers accompanying a young man wearing a gray T-shirt and red shorts into a waiting car. His head was bowed and his hair was close-cropped.

The Justice Department did not say what charges Teixeira would face, although they will likely involve criminal charges of willfully retaining and transmitting national defense information.

Brandon Van Grack, a former Justice Department national security prosecutor now with the law firm Morrison Foerster, said the likely charges could carry up to 10 years’ imprisonment, even if Teixeira did not intend to cause harm.

“I think this is someone who is facing on the higher end of exposure for years in prison… because the leaks were so damaging,” Van Grack said.

A spokesperson for the U.S. Attorney’s Office in Boston said Teixeira is expected to make his initial appearance on Friday.

The documents were posted online on a social media site in March, and perhaps earlier, but did not spread widely until last week.

U.S. officials have been hunting for the person responsible, while assessing and seeking to limit damage from the release of the intelligence reports, which reveal purported details of Ukrainian military vulnerabilities and information about allies including Israel, South Korea and Turkey.

It is believed to be the most serious security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010.

The Justice Department opened a formal criminal probe last week, after a referral from the Department of Defense. The leak was a “deliberate, criminal act,” the Pentagon said on Thursday, adding that the military had taken steps to review distribution lists and ensure people receiving information had a need to know.

Reuters has reviewed more than 50 of the documents, labeled “Secret” and “Top Secret”, but has not independently verified their authenticity. The number of documents leaked is likely to be more than 100.

President Biden, who is on a three-day tour of Ireland, said he was not overly concerned about the leak.

“I’m concerned that it happened, but there is nothing contemporaneous that I’m aware of,” Biden told reporters.

A man named Jack Teixeira was promoted to Airman 1st Class in July, according to a post on the 102nd Intelligence Wing’s official Facebook page. The unit did not immediately return an email seeking comment.

A number of countries have questioned the veracity of some of the leaked documents, including Britain, which said there was “a serious level of inaccuracy” in the information.

U.S. officials believe most of the materials are genuine. Some, however, appear to have been altered to show inflated estimates for Ukrainian battlefield casualties in the war with Russia as well as understated numbers for Russian forces.

Other documents include an assessment that France is likely to struggle to achieve security goals in west and central Africa, and information about Brazilian officials’ plan to visit Moscow in April to discuss a Ukraine mediation scheme.

According to a leaked Pentagon document, Serbia has agreed to supply arms to Kyiv or has sent them already, despite the country’s professed neutrality in the Ukraine war.

The New York Times first reported Teixeira’s identity earlier on Thursday, describing him a National Guardsman who led Thug Shaker Central, an online group where about 20 to 30 people shared their love of guns, racist memes and video games. The Times cited interviews and documents it reviewed.

— Reuters. Edited for style.