The U.S. 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.

Each of the counts carries sentences of up to 10 years in prison.

"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 U.S. Attorney General Merrick Garland while announcing the indictment.

"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 U.S. law and endangered our national security," he added.

The charges are in addition to the two counts filed by prosecutors against Teixeira after his arrest in April.

Teixeira is suspected of posting the documents to a private chat group on the social media platform Discord. Some of the files later appeared on other sites, including Twitter, 4Chan and Telegram.

The documents, which soon spread across the internet, showed Ukraine's defense amid its conflict with Russia, internal deliberations within the South Korean government about decisions on potential indirect arms transfer to Ukraine, U.S. assessment on support from Mossad – Israel's spy agency – for mass protests against a judicial reform in the Jewish state, among others.

It was the biggest such breach since the 2013 dump of National Security Agency documents by Edward Snowden.

For months, officials from multiple U.S. 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.

(With input from agencies)