Air National Guardsman Jack Teixeira pleaded guilty on Monday to leaking highly classified military documents about the war in Ukraine and other national security secrets.
The admissions were part of a deal with prosecutors under which he will serve at least 11 years in prison.
Teixeira, 22, pleaded guilty to six counts of wilful retention and transmission of national defence information under the Espionage Act, nearly a year after he was arrested over the most consequential national security leak in years.
The Massachusetts guardsman admitted illegally collecting some of the nation's most closely guarded secrets and sharing them with other users on Discord, a social media platform popular with people playing online games.
US District Judge Indira Talwani scheduled sentencing for September and said she would decide then whether to formally accept the agreement, which calls for a prison sentence between 11 and nearly 17 years.
Prosecutors said they plan to seek the high end of that range.
“Mr Teixeira callously disregarded the national security of the United States and he betrayed his solemn oath to defend the country, and the trust of the American people he swore to protect,” Matt Olsen, assistant attorney general for national security, said after the hearing.
The stunning security breach raised alarm over America’s ability to protect its most closely guarded secrets and forced the administration of President Joe Biden to desperately try to contain diplomatic and military fallout.
The leaks embarrassed the Pentagon, which tightened controls to protect classified information and disciplined members found to have intentionally failed to take required action about Teixeira’s suspicious behaviour.
The classified material included highly sensitive US military assessments, including on the war in Ukraine.
The leak is considered to be the most serious US national security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010.
Michael Bachrach, a lawyer for Teixeira, told reporters they will push for a sentence of 11 years.
Mr Bachrach described Teixeira as a “kid,” adding that the defence will show at sentencing that his youth played a significant role in his conduct.
“He is significantly remorseful for his conduct," he said. "He has accepted full responsibility for his conduct."
