An undated picture shows Jack Douglas Teixeira, a 21-year-old member of the US Air National Guard, who was arrested by the FBI, over his alleged involvement in leaks online of classified documents, posing for a selfie at an unidentified location. (PHOTO / SOCIAL MEDIA WEBSITE / REUTERS)

WASHINGTON – The US air national guardsman accused of leaking classified documents involving intelligence on the Russia-Ukraine conflict and others was indicted Thursday by a federal grand jury on charges of unlawful disclosure of national defense information.

Jack Teixeira, 21, of the Massachusetts Air National Guard, was indicted in Boston on six counts of willful retention and transmission of classified information relating to the national defense, according to a statement issued by the Department of Justice.

The leaked materials showed the United States' continued efforts to eavesdrop not only on countries it regarded as foes, but even on those that have long been its allies as well

"As laid out in the indictment, Jack Teixeira was entrusted by the United States government with access to classified national defense information – including information that reasonably could be expected to cause exceptionally grave damage to national security if shared," said US Attorney General Merrick Garland while announcing the indictment.

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"Teixeira is charged with sharing information with users on a social media platform he knew were not entitled to receive it. In doing so, he is alleged to have violated US law and endangered our national security," he added.

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The young airman was arrested in April and has been detained ever since. He was alleged to have leaked classified documents from the Department of Defense, of which the content pertained to Ukraine's defense amid its conflict with Russia, internal deliberations within the South Korean government about decisions on potential indirect arms transfer to Ukraine, US assessment on support from Mossad – Israel's spy agency – for mass protests against a judicial reform in the Jewish state, among others.

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The leaked materials, which were first shared by Teixeira with his friends on Discord – an online chatting app mostly attracting video gamers – before being more widely spread on the Internet, showed the United States' continued efforts to eavesdrop not only on countries it regarded as foes, but even on those that have long been its allies as well.

For months, officials from multiple US government agencies — including the National Security Council, the Department of State as well as the Pentagon – have scrambled to contain the fallout of the scandal by both refusing to confirm the veracity of the leaked information and minimizing the incident's impact on cooperation between Washington and its allies.