Teixeira’s arrest on Thursday came “in connection with an investigation into alleged unauthorised removal, retention and transmission of classified national defense information”, according to Garland.
Pentagon officials said they were still assessing the scale and scope of the leak, which appeared to show classified information about the war in Ukraine and US allies.
Officials have repeatedly warned, however, that some of the documents had been doctored and may be used in disinformation campaigns.
While authorities have offered few details about Teixeira, details about his life started to appear in US media in the wake of his arrest.
Video showed heavily armed officers in Massachusetts accompanying a young man wearing a gray T-shirt and bright red shorts into a waiting car. His head was bowed and his hair was close-cropped.
The New York Times reported that Teixeira was a member of the intelligence wing of the state national guard. A post on the unit’s official Facebook page noted that Teixeira was among a group of members who had been promoted to Airman First Class in July 2022.
The Air Force later said Teixeira joined the Air National Guard in September 2019. His official job title was “cyber transport systems journeyman”. The role is tasked with maintaining the underlying infrastructure of the Air Force’s “vast, global communications network”.
“Whether it’s repairing a network hub at a stateside base or installing fiber-optic cable at a forward installation overseas, these experts keep our communications systems up and running and play an integral role in our continuing success,” a description of the role on the Air Force’s website said.
The Times and other US media have also identified Teixeira as the leader of the small gaming chat group on the Discord platform, where the leaked documents first emerged.
The Washington Post had previously reported that the online group formed in 2020, amid the coronavirus pandemic.
The chat group, composed of about two dozen young men and boys, bonded over their love of guns, military gear and religion. Membership was invitation only, according to the newspaper.
One member said: “We all grew very close to each other, like a tightknit family … We depended on each other.”
At the centre of that family was Teixeira, whom members knew as OG. He was considered the group’s leader.
Teixeira was widely admired for his bravado, with members saying he would post videos of himself at a gun range, including one where he uttered racial and anti-Semitic slurs before opening fire, the Post reported. Offensive jokes and memes were common on the site.
At some point last year, members told the newspaper, Teixeira began posting what appeared to be re-written transcripts of classified information, which he touted as available through his work. The transcriptions included definitions of military jargon and other annotations.
He regularly lectured the group on the importance of staying abreast of current affairs and appeared to post the transcribed intelligence to inform other members, according to Post.
Teixeira later switched to posting photos of the documents, although a timeline of when that began was not immediately clear.
Bellingcat, an open source investigative news site, had previously reported that the documents first emerged on Discord at latest by March, but they may have been posted as far back as January.
The classified materials gradually made their way to other sites, including the imageboard 4Chan, as well as to more mainstream social platforms. Discord has said it is cooperating with investigators.
Reached at her Massachusetts home on Thursday, Teixeira’s mother Dawn told the New York Times that her son had been working overnight shift at a base on Cape Cod.
She also explained that in recent days he had changed his phone number. He arrived at the house in red pickup truck shortly before he was taken into custody.
In Thursday’s statement, Attorney General Garland said Teixeira would have an initial appearance at the US District Court for the District of Massachusetts but did not give a date.
US media began widely reporting on the leak last week, sending the administration of President Joe Biden scrambling to contain the fallout from the documents, which showed vulnerabilities in Ukraine’s air defense capabilities and exposed private assessments by allies on an array of intelligence matters.
The documents have also roiled close US allies. Some appeared to show that Egypt planned to sell weapons to Russia, in a deal it planned to keep secret from Washington. Another seemingly showed that Russian operatives were building a closer relationship with the United Arab Emirates, while a third document indicated that South Korean leaders were hesitant to ship artillery shells to Ukraine.
The governments involved in those claims have all denied the information in the documents.
Earlier on Thursday, Biden told reporters: “I’m not concerned about the leak. I’m concerned that it happened, but there’s nothing contemporaneous that I’m aware of that is of great consequence.”
On Thursday, Department of Defence spokesperson Pat Ryder said his agency would not address specifics of the leaked documents, as they had not been officially declassified.
“I will highlight that as a matter of long standing policy, just because classified information may be posted online or elsewhere does not mean it has been declassified by a classification authority,” he said.
“We’re just not going to discuss or confirm classified information due to the potential impact on national security, as well as the safety and security of our personnel, and those of our allies and our partners.”