The mastermind behind one of America’s most damaging spy rings has reportedly died.

John A. Walker Jr., 77, was sentenced in 1986 to two life terms plus 10 years for selling U.S. secrets to the Soviets as a cryptologist in the Navy and after he retired. He died Thursday in a federal prison in North Carolina, according to prison officials, less than nine months away from his expected release due to federal parole guidelines at the time, PilotOnline.com reports.

Walker eventually enlisted his brother Arthur, his son Michael and a Navy friend, Jerry Whitworth, into one of the biggest security breaches in the nation’s history.

Robert Hunter, the FBI agent who arrested Walker, characterized him as one of the most treacherous men he’d ever met.

“I think the man was pure evil,” Hunter told PilotOnline.com.

Walker’s foray into espionage began in 1967 while working at what’s now the Norfolk Naval Station, where he stole and later sold “key cards” to the Soviet Union, allowing its intelligence officers to unlock more than 1 million top-secret and classified messages.

Walker agreed to plead guilty as part of a deal with prosecutors to obtain a lighter sentence for his son. 

Former Navy Seaman Michael L. Walker served 15 years in prison and was released in 2000.

The brother, retired Navy lieutenant commander Arthur Walker, died at the Butner hospital in July.

The fourth member of the spy ring, Ex- Navy Chief Petty Officer Jerry A. Whitworth was convicted in 1986 and later sentenced to a total of 365 years. A database of federal inmates shows that Whitworth, now 75, is incarcerated at the Federal Penitentiary in Atwater, California.

The Associated Press contributed to this report.