US Attorney-General Merrick Garland said the Federal Bureau of Investigation (FBI) arrested Jack Teixeira “in connection with an investigation into alleged unauthorised 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 “authorised law enforcement activity at a residence in North Dighton, Massachusetts”.

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.

FORMAL PROBE

US officials have been hunting for weeks for the person responsible and assessing damage from the release of the intelligence reports, which first appeared on social media sites in March and purportedly reveal details of Ukrainian military vulnerabilities and information about allies including Israel, South Korea and Turkey.

The Department of Defense referred the matter to the Justice Department, which opened a formal criminal probe last week. The Pentagon is assessing the damage.

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 over 100.

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

“There’s a full-blown investigation going on, as you know, with the intelligence community and the Justice Department, and they’re getting close but I don’t have an answer,” Biden told reporters.

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

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.

DOCUMENTS ON UKRAINE, ISRAEL, SOUTH KOREA

Some of the most sensitive leaked details are purportedly related to Ukraine’s military capabilities and shortcomings, and information about US allies.

Biden said earlier on Thursday that investigators were closing in on the source of the online leaks in what is believed to be the most serious security breach in years.

The New York Times earlier reported that Teixeira (21) was 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.

(Reporting by Sarah N. Lynch and Idrees Ali; Additional reporting by Steve Holland in Dublin, Katharine Jackson, Susan Heavey, Jasper Ward and Raphael Satter in Washington, and Michelle Nichols at the United Nations; Writing by Alistair Bell and Patricia Zengerle; Editing by Don Durfee, Matthew Lewis and Daniel Wallis.)