WASHINGTON, April 13 (Reuters) - The FBI on Thursday 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.

Federal agents in an armored car and military gear swooped in on Teixeira, dressed in gym shorts, a T-shirt and trainers, at his home in Dighton, Massachusetts, a mostly wooded town of 8,000 about 50 miles (80 km) south of Boston.

The arrest comes a week after the leaks first became widely known, setting Washington on edge about the damage they may have caused. The episode embarrassed the U.S. by revealing its spying on allies and purported Ukrainian military vulnerabilities.

The leak of documents, posted largely on social media sites, was 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.

Teixeira was an airman 1st class at Otis Air National Guard Base in Massachusetts, according to his service record. He joined the Air National Guard in 2019 and worked as a "Cyber Transport Systems Journeyman," or an IT specialist.

Attorney General Merrick Garland told reporters Teixeira was wanted "in connection with an investigation into alleged unauthorized removal, retention, and transmission of classified national defense information."

The FBI said its agents had conducted "authorized law enforcement activity at a residence in North Dighton, Massachusetts."

Aerial news video showed Teixeira with his hands laced behind his head, walking backward toward the armored car with one officer watching from the turret. He was handcuffed and placed in the back of the vehicle. Garland said he was taken into custody "without incident."

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.

Although the leak only garnered widespread attention after an April 6 story in the New York Times, journalists have found evidence that the documents – or at least some of them – had been floating around on social media as far back as March or even January.