The FBI has arrested a 21-year-old air national guardsman in Massachusetts suspected of being responsible for the leak of US classified defence documents that laid bare military secrets and upset Washingon’s relations with key allies. Jack Teixeira was arrested at his home in the town of North Dighton by FBI agents. Helicopter news footage showed a young man with shorn dark hair, an olive green T-shirt and red shorts being made to walk backwards towards a team of agents standing by an armoured vehicle dressed in camouflage and body armour, pointing their rifles at him. In Washington, the US attorney general, Merrick Garland, confirmed the arrest, saying Teixeira was being held “in connection with an investigation into alleged unauthorized removal, retention and transmission of classified national defence information”. Garland’s use of language suggests Teixeira will be facing charges under the Espionage Act. Each charge under the act can carry an up to 10-year prison term, and prosecutors could treat each leaked document as a separate count in his indictment. He could be facing a very long jail sentence. Garland said the air national guardsman would make an initial appearance at the Massachusetts district court in Boston. Airman first class Teixeira was in the 102nd Intelligence Wing of the Massachusetts air national guard under the duty title of “cyber transport systems journeyman”, responsible for keeping the internet working at airbases. He joined the guard in 2019. Teixeira is believed to have been the leader of an online chat group where hundreds of photographs of secret and top-secret documents were first uploaded, from late last year to March. The online group called itself Thug Shaker Central, made up of 20 to 30 young men and teenagers brought together by an enthusiasm for guns, military gear and video games. Racist language was a common feature of the group. Former members of Thug Shaker Central have told the investigative journalism organisation Bellingcat, the Washington Post and the New York Times that the documents were shared in an apparent attempt to impress the rest of the group, rather than to achieve any particular foreign policy outcome. Speaking in Ireland, Joe Biden sought to play down the impact of the breach. “I’m not concerned about the leak,” Biden insisted. “I’m concerned that it happened. But there’s nothing contemporaneous that I’m aware of that’s of great consequence.” The Guardian has seen about 50 of the documents. But there are signs that many more were first posted on Thug Shaker Central. The New York Times said it had seen about 300 of the documents, only a fraction of which have so far been reported, indicating the national security damage could be worse than has so far been acknowledged. One of the ways the leak could have an impact on US security is if it makes allies wary of sharing intelligence. The Polish prime minister, Mateusz Morawiecki, denied it would have affect his country’s confidence in Washington’s ability to keep secrets. “I’m not going to think twice,” Morawiecki told the Guardian at an Atlantic Council event in Washington. “I believe failures happen and mistakes happen, but we have to be as close as possible to our allies in western Europe and the United States. We have to unite on this front as well.” The spokesman for the Pentagon, Brig Gen Patrick Ryder said: “We have rules in place. Each of us signs a nondisclosure agreement, so all indications are that this is a criminal act.” Part of the inquest into the leak will examine how a 21-year-old air national guardsman in Massachusetts could have access to top-secret material vital to US and allied security interests, including battlefield deployments in Ukraine. The Pentagon said on it was reviewing its policies on safeguarding classified material, including updating distribution lists and assessing how and where intelligence is shared. “It’s important to understand that this is not just about DoD [the defence department]. This is about the US government,” Ryder said. “This is about how we protect and safeguard classified information. We do have strict protocols in place, so any time there is an incident there’s an opportunity to review that and refine it.” In North Dighton, the woman believed to be Jack Teixeira’s mother, Dawn Dufault, previously Dawn Teixeira, and her husband, Tom Dufault, own a nursery called Bayberry Farm and Flower Co. Calls to the company went to voicemail on Thursday. A message said the business is closed this week. The company’s Facebook page had made mention of Jack Teixeira in June 2021. “Jack is on his way home today, tech school complete, ready to start his career in the Air National Guard!” a message said, under a photograph of a “Welcome home” balloon. In December 2020, the company posted congratulating “Jack” on his 19th birthday, beneath a picture of a person in military-type dress. Among some of the newly reported leaked materials are documents showing knowledge of infighting between Russian intelligence and the defence ministry. In one document reported by the New York Times, US officials describe how the Federal Security Service (FSB) had “accused the defence ministry of trying to cover up the extent of Russian casualties in Ukraine”. The FSB said the official statistics did not include the dead and wounded from the national guard or two significant militias involved in combat, the Wagner mercenary force and fighters fielded by the Chechen republic’s warlord leader, Ramzan Kadyrov. The US intelligence assessment was that the spat demonstrated “the continuing reluctance of military officials to convey bad news up the chain of command”. According to the teenage member of the Thug Shaker group interviewed by the Washington Post, their leader, who he referred to as OG but is now thought to be Teixeira, “had a dark view of the government”, portraying the government, and particularly law enforcement and the intelligence agencies, as a repressive force. He ranted about “government overreach”. The teenage group member was in touch with the man he called OG in the days leading up to his arrest, and said he “seemed very confused and lost as to what to do”. “He’s fully aware of what’s happening and what the consequences may be,” he said. “He’s just not sure on how to go about solving this situation … He seems pretty distraught about it.” In his final message to his fellow group members, the fugitive told them to “keep low and delete any information that could possibly relate to him”, including any copies of the classified documents.