The FBI used billing records from an internet social media app to unmask US airman Jack Teixeira as a suspect over the leak of top-secret military documents, it has been revealed.
Court documents made public on Friday showed investigators identified the Massachusetts Air National Guardsman, 21, using information on the Discord chat platform.
The leak is believed to have started on its servers, which are frequently used by people who play online games.
Live blog: Outside court as US secrets leak suspect Jack Teixeira is charged
Classified information is said to range from briefing slides mapping out military positions in Ukraine to consideration of what circumstances Russian President Vladimir Putin might use nuclear weapons.
Discord said it was cooperating with the bureau, according to an FBI affidavit unsealed on Friday.
The document says the FBI interviewed an unnamed person familiar with Teixeira’s posts on Monday.
Teixeira is said to be “at the heart” of a private channel called Thug Shaker Central, allegedly posting about guns, games, jokes and memes, people who say they chatted with him have claimed.
The anonymous person told the FBI a username linked to Teixeira began posting what appeared to be classified information last December.
The affidavit suggests Teixeira had been typing out documents in his possession but started taking them home and photographing them after becoming “concerned he may be discovered making the transcriptions in the workplace”.
However, others said the user, nicknamed “the O.G” – also known as “original” or a way to describe someone who is the founder or influential in a group – had been posting documents he said were classified in the group for months.
The O.G started posting images of documents because they were annoyed others were not taking them seriously, it is alleged.
Teixeira was detected on 6 April, according to the affidavit – the same day the New York Times first published a story about the classified documents being leaked.
He was arrested on Thursday by heavily armed tactical agents after a week-long investigation into the disclosure of the highly sensitive government records.
Read more about Jack Teixeira:
Who is the 21-year-old national guardsman suspected of US intelligence leaks?
How did someone so young have access to such sensitive files?
US under pressure as Pentagon vows to tighten security
Family members sob during court appearance
The 21-year-old appeared before a federal magistrate judge at a court in Boston on Friday, charged with possessing unclassified documents pertaining to national security and possessing national defence materials.
Family members were seen crying and consoling each other before the defendant was brought into the courtroom.
Wearing a brown jumpsuit, he was ushered into the court by US marshals.
The judge ordered him to remain behind bars until a detention hearing on 19 April.
The leaked documents have not been individually authenticated by officials.
A US Defence Department official said Teixeira could have had a higher level of security clearance due to being an information technology specialist responsible for military communications networks.
The Pentagon will now conduct a review of its intelligence access, accountability, and control procedures to prevent such a leak from happening again, said US Defence Secretary Lloyd Austin.