(CNN) -

John Walker Jr., a former U.S. Navy officer convicted of spying decades ago for the Soviet Union, has died in federal prison, according to the U.S. government.

Walker, 77, died Thursday at a federal correctional facility in Butner, North Carolina, the Federal Bureau of Prisons said. The agency's website indicated Walker was scheduled to be released on parole within the year.

Authorities said Walker stole, then sold, codes to help unlock encrypted Navy messages, which allowed the Soviets to monitor American military assets.

Walker pleaded guilty to running a spy ring that included his brother, son and good friend. The spying went on from 1968 to 1985, when he was arrested by the FBI -- caught in part due to a tip from his ex-wife.

Walker joined the Navy in 1955 as radioman and served on board six different vessels in the first decade of his career.

The impersonal nature of serving on large vessels took a personal toll on him; Walker became even more politically disillusioned after President John F. Kennedy's assassination, and eventually started selling secrets to Moscow, according to a profile of Walker in Naval History magazine.

"Walker's information not only provided us with ongoing intelligence, but helped us over time to understand and study how your military actually thinks," Soviet spy chief Boris Solomatin told author Pete Earley.

U.S. officials later said Walker's actions did significant damage to national security.

Walker's spying gave the Soviet Union "access to weapons and sensor data and naval tactics, terrorist threats, and surface, submarine, and airborne training, readiness and tactics," the late U.S. Secretary of Defense Caspar Weinberger said.

Walker's brother, Arthur -- a retired Navy lieutenant commander -- died last month in the same prison. He was 79.

Michael Walker, John's son, was released in 2000 after serving 15 years in prison. The youngest Walker also served in the Navy.