HANSCOM AIR FORCE BASE, Massachusetts (Reuters) - Jack Teixeira, a member of the Massachusetts Air National Guard who pleaded guilty to leaking classified military documents on topics including the war in Ukraine, is slated to appear for a military justice proceeding on Tuesday to face new charges.

Teixeira, 22, was arrested in April 2023 and accused of carrying out one of the most serious U.S. national security breaches in years.

He pleaded guilty in March to charges brought by the U.S. Department of Justice after reaching a deal that called for him to serve at least 11 years in prison.

But the case in civilian court left open the possibility of the Air Force pursuing separate charges against him in the military justice system, which it announced on May 1 it would do over alleged misconduct related to Teixeira's duties.

He faces military charges of obstructing justice and failing to obey a lawful order. Those charges carry a maximum sentence of 10-1/2 years in prison if Teixeira is convicted.

Prosecutors at Tuesday's hearing at Hanscom Air Force Base, about 20 miles (32 kilometers) northwest of Boston, will present evidence to a neutral officer to determine whether the evidence is sufficient for the case to proceed to trial by court-martial.

A military lawyer for Teixeira could not be identified.

Before his arrest at his mother's house in North Dighton, Massachusetts, Teixeira had been an airman 1st class at Otis Air National Guard Base on Cape Cod in Massachusetts, where he worked as a cyber defense operations journeyman, or information technology support specialist.

Despite being a low-level airman, Teixeira held a top-secret security clearance, and starting in January 2022 began accessing hundreds of classified documents related to topics including Russia's invasion of Ukraine, according to federal prosecutors.

Under the username "TheExcaliburEffect," Teixeira shared classified information on the messaging app Discord in private servers - a kind of chat room - while bragging he had access to "stuff for Israel, Palestine, Syria, Iran and China."

The leaked documents held highly classified information on allies and adversaries, with details ranging from troop movements in Ukraine to Israel's Mossad spy agency.

He is slated to be sentenced in the Justice Department's case on Sept. 27.

(Reporting by Nate Raymond in Boston, Editing by Alexia Garamfalvi and Bill Berkrot)

By Nate Raymond