A 21-year-old member of the U.S. Air Force National Guard suspected of leaking highly classified military intelligence records online made his initial appearance before a federal judge in Boston on Friday.

Jack Douglas Teixeira of North Dighton, Massachusetts, was arrested by the FBI at his home on Thursday without incident.

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.

U.S. officials are still assessing the damage done by the leaks, which included records showing purported details of Ukrainian military vulnerabilities and information about allies including Israel, South Korea and Turkey.

Although charging documents for Teixeira have not yet been made public, U.S. Attorney General Merrick Garland said on Thursday that he was being arrested in connection with "an investigation into alleged unauthorized removal, retention, and transmission of classified national defense information."