Massachusetts Air National Guardsman Jack Teixeira agreed Monday that he caused one of the most extraordinary leaks of national defense secrets in years and agreed to accept a prison sentence of 16 years — what could be the longest sentence in an unlawful retention case.

According to the signed plea agreement filed with the court, Teixeira, 22, agreed to plead guilty to all six counts charging him with willful retention and transmission of national defense information. In exchange, prosecutors agreed not to charge him with additional counts under the Espionage Act.

Teixeira “accessed and printed hundreds of classified documents” and posted images of them on Discord prior to his arrest last April, a prosecutor said Monday during a hearing in Boston federal court, where the Air National Guardsman pleaded guilty.

Teixeira entered court in handcuffs and an orange jumpsuit with the initials PCCF on the back, for Plymouth County Correctional Facility, and appeared to smile at his father, who was seated in the second row.

He stood with defense attorney Michael Bachrach before Judge Indira Talwani, identified himself and answered basic questions. His close-cropped dark hair was neatly combed on top and shaved on the bottom half of his head. 

Talwani asked Teixeira whether he understood he was agreeing to serve as much as 16 years in prison and no fewer than 11 years.

“Yes your honor, I understand,” Teixeira said, leaning down to speak into a microphone at the defense table.

“How do you now plead to counts one, two, three, four, five and six? Guilty or not guilty?” the clerk asked. “Guilty,” Teixeira responded.

As part of his plea agreement, Teixeira must sit for a debrief with the Defense Department and the Justice Department and give back any sensitive materials that might remain in his possession.

Teixeira’s sentencing is set for Sept. 27.

Federal prosecutors made clear Teixeira had no business peering at classified information because his low-level job did not require it.

“The defendant’s job was to troubleshoot computer workstations,” Assistant United States Attorney Jason Casey said. 

Still, Casey said, Teixeira accessed “hundreds” of classified documents inside the secure facility where he worked and “purposefully removed classified documents and information despite admonishments from his superiors to stop.”

Teixeira posted the material on Discord using the moniker “theexcalibereffect” and “took steps to conceal his unlawful activity,” Casey said.

Without mentioning specifics, federal prosecutors said Teixeira exposed information about the compromise by a foreign adversary of certain accounts belonging to a U.S. company and information about equipment the U.S. was sending to Ukraine, how it would be transferred and how it would be used upon receipt. Prosecutors said he also posted material about troop movements in Ukraine, a plot by a foreign adversary to attack U.S. forces abroad, and Western deliveries of supplies to the Ukrainian battlefield.

“You knew it was marked classified, correct?” Talwani asked Teixeira. “Yes your honor,” Teixeira responded.

Casey quoted Teixeira as telling members of his Discord chat group he was “breaking a bunch of UD regs,” referring to unauthorized disclosure regulations, though he also boasted, according to prosecutors, “No one knows anything incriminating about me.”

Teixeira, who was 21 years old when he was arrested, had pleaded not guilty in June 2023 to the six counts of willful retention and transmission of national defense information.

Teixeira enlisted in the Air National Guard in 2019, according to his service record, and had top secret security clearance since 2021, according to the Department of Justice.

He purportedly began posting classified documents online in January 2022, according to the Justice Department.

In December, 15 airmen were disciplined — including with removal of their command — for failing to take proper action when they became aware of Teixeira’s intelligence-seeking activities, according to a broad Air Force investigation.

The internal investigation placed blame on Teixeira for the unauthorized disclosure of classified information, but it found that members of his unit failed to take necessary steps such as adequately inspecting areas under their command and giving inconsistent guidance for reporting security incidents.

ABC News’ Alexander Mallin contributed to this report.