Air National Guardsman Jack Teixeira will plead guilty to leaking of classified military information in a case from last year that was considered one of the most serious U.S. national security breaches in recent years, a spokesperson for the U.S. Attorney's office in Boston said.

WHO IS JACK TEIXEIRA, THE AIR NATIONAL GUARDSMAN BEHIND LEAKED CLASSIFIED DOCUMENTS?

Prosecutors say Teixeira, who was 21 at the time of his arrest in June last year, leaked classified documents to a group of gamers on the messaging app Discord.

Jack Teixeira Boston Federal Courthouse

A general view of federal courthouse where Jack Teixeira, a member of the U.S. Air Force National Guard suspected of leaking highly classified U.S. documents, makes his initial appearance in Boston, Massachusetts, U.S., April 14, 2023. (REUTERS/Lauren Owen Lambert)

The classified material included highly sensitive U.S. military assessments, including on the war in Ukraine.

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The leak was considered the most serious U.S. national security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010.