What punishment could the US leaker face?
Jack Teixeira faces a lengthy prison sentence after being charged with unauthorised transmission of national defence information and unauthorised removal of classified documents or material.
Mr Teixeira was told in court he faced up to 15 years in jail if convicted of both violations.
He was detained on Friday ahead of a court hearing scheduled for 19 April.
The Pentagon said his actions posed a "very serious" national security risk.
The 21-year-old military cyber-specialist is suspected of leaking sensitive US documents labelled top secret, secret and confidential that could cause damage to US national security.
The Pentagon said it believed the leak was a deliberate criminal act.
Mr Teixeira, who made his first appearance in court on Friday in Boston, is a member of the intelligence wing of the Massachusetts Air National Guard. The charge sheet says he was given top-secret security clearance in 2021 and since February this year had the title of "Cyber Defence Operations Journeyman".
Merrick Garland, the US attorney general, said Mr Teixeira faced charges under the 1917 Espionage Act.
The first charge of unauthorised retention and transmission of national defence information carries a 10-year maximum sentence.
For the second charge of unauthorised removal and retention of classified documents or material there is a five-year maximum jail term.
Steven Stransky, an attorney who previously served as senior counsel to the Department of Homeland Security's Intelligence Law Division, said the act was "very old and outdated" and "essentially criminalises an individual collection, disclosure or potentially re-disclosure, of national defence information".
He added that while the term "national defence information" is only vaguely defined in the law, it broadly means information that "can injure the United States or put us at a disadvantage vis-a-vis another third country".
Several spies and people who have shared classified information with the press and the public have been charged since the law was first enacted in 1917, albeit infrequently.
It has historically largely been applied to Americans found spying for foreign countries, such as Julius and Ethel Rosenberg, who were executed in 1953 after being found guilty of passing nuclear secrets to the Soviet Union. The Espionage Act authorised the death penalty for espionage "in time of war" - and at the time, the US was involved in the Korean war.
More recently, the Espionage Act has been applied to whistleblowers and people who have leaked sensitive information, including Wikileaks source Chelsea Manning and ex-CIA contractor Edward Snowden.
When the Espionage Act was originally passed, it stipulated prison sentences of 20 years or less and fines of up to $10,000 per charge.
"This is someone who is facing on the higher end of exposure for years in prison... because the leaks were so damaging," Brandon Van Grack, a former Justice Department national security prosecutor now with the law firm Morrison Foerster, told Reuters news agency.
"There's certainly criminal charges that could attach to it [the Espionage Act] and there's financial penalties as well," Mr Stransky said. "If the justice department is pursuing a criminal violation for the Espionage Act, they're most often looking for jail sentencing as a way to deter these types of future actions."
The Pentagon press secretary indicated it would treat the leak extremely seriously, saying it was "a deliberate criminal act". A Pentagon spokesperson said that "stringent" guidelines were in place for employees and that "anyone who violates those rules is doing so wilfully".
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