BOSTON — Massachusetts Air National Guard member Jack Teixeira pleaded guilty on Monday in federal court to leaking highly classified military documents about Russia’s war in Ukraine and other national security secrets.

Teixeira pleaded guilty to six counts of willful retention and transmission of national defense information under the Espionage Act nearly a year after he was arrested in the most consequential national security leak in years.

The stunning breach raised alarm over America’s ability to protect its most closely guarded secrets and forced the Biden administration to scramble to try to contain diplomatic and military fallout. The leaks embarrassed the Pentagon, which tightened controls to safeguard classified information and disciplined members found to have intentionally failed to take required action about Teixeira’s suspicious behavior.

Teixeira, 22, admitted illegally collecting military secrets and sharing them with other users on Discord, a social media platform popular with people playing online games. Prosecutors plan to seek nearly 17 years in prison for him, according to the plea agreement.

Teixeira, who was part of the 102nd Intelligence Wing at Otis Air National Guard Base in Massachusetts, worked as a cyber transport systems specialist, essentially an information technology specialist responsible for military communications networks.

Authorities said he first typed out classified documents he accessed and then began sharing photographs of files that bore SECRET and TOP SECRET markings. The leak exposed to the world unvarnished secret assessments of Russia’s war in Ukraine, the capabilities and geopolitical interests of other nations and other national security issues.

Teixeira remains in the Air National Guard in an unpaid status, an Air Force official said.

Teixeira has been behind bars since his April arrest. The judge denied his request for release from jail last year after prosecutors revealed he had a history of violent rhetoric and warned that U.S. adversaries who might be interested in mining Teixeira for information could facilitate his escape.

Prosecutors have said little about a motive. But members of the Discord group described Teixeira as someone looking to show off, rather than being motivated by a desire to inform the public about U.S. military operations or to influence American policy.

Prosecutors have said Teixeira continued to leak government secrets even after he was warned by superiors about mishandling and improper viewing of classified information. In one instance, Teixeira was seen taking notes on intelligence information and putting them in his pocket.

The Air Force inspector general found that members “intentionally failed to report the full details” of Teixeira’s unauthorized intelligence-seeking because they thought security officials might overreact. For example, while Teixeira was confronted about the notes, there was no follow-up to ensure the notes had been shredded and the incident was not reported to security officers.

It was not until a January 2023 incident that the appropriate security officials were notified, but even then security officials were not briefed on the full scope of the violations.