A 21-year-old member of the US Air Force National Guard accused of leaking highly classified military intelligence records online made his initial appearance before a federal judge in Boston on Friday to face charges he unlawfully removed and retained classified materials.

Jack Douglas Teixeira of North Dighton, Massachusetts, who was arrested by a team of heavily armed FBI agents at his home on Thursday, appeared in federal court wearing a brown khaki jumpsuit.

At the hearing, Boston’s top federal national security prosecutor, Nadine Pellegrini, requested that Teixeira be detained pending trial, and a detention hearing was set for Wednesday.

The leaked classified documents at the heart of the investigation were posted online on a social media website in March and perhaps earlier, but news of their existence did not come to light until it was reported by the New York Times last week.

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. In the WikiLeaks case, the leaker - US Army Private First Class Chelsea Manning - was sentenced to 35 years in prison. Democratic President Barack Obama later commuted her sentence.

US officials are still assessing the damage done by the leaks, which included records showing purported details of Ukrainian military vulnerabilities and embarrassed Washington by revealing its spying on allies.

In a criminal complaint made public on Friday, Teixeira was charged with unlawfully copying and possessing classified defense records. Each offense can carry up to 10 years in prison.

He was also charged with another offense which makes it a crime for an employee of the United States to knowingly remove classified records to an unauthorized location.

In a sworn statement, an FBI agent said that Teixeira had held a top secret security clearance since 2021, and that he also maintained sensitive compartmented access to other highly classified programmes.

Since May 2022, the FBI said Teixeira has been serving as an E-3/airman first class in the US Air Force National Guard and has been stationed at Otis Air National Guard Base in Massachusetts.

Teixeira only spoke twice during the brief proceeding, answering “yes” when asked if he understood his right to remain silent.

He also confirmed that he had filled out a financial affidavit, which the judge said shows he will qualify to be represented by a federal public defender.

The Justice Department opened a formal criminal probe last week into the current leaks, 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.

It was not clear whether Teixeira had yet secured legal representation.

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.

The investigative news outlet Bellingcat, the Washington Post and the New York Times have traced the documents’ earliest appearance to a defunct server on the instant messaging site Discord. In a chat group on the site, Teixeira went by the handle OG and was admired by the group’s mostly young members, who shared a love for guns and military gear.