Charles Harrelson, 69, was found unresponsive in his cell on the morning of March 15, said Felicia Ponce, a Bureau of Prisons spokeswoman in Washington.
Fremont County Coroner Dorothy Twellman said an autopsy showed Harrelson had severe coronary artery disease. She said he probably died in his sleep. "It appears it was very sudden."
Charles Harrelson was convicted of murder in the May 29, 1979, slaying of U.S. District Judge John Wood Jr. outside his San Antonio, Texas, home. Prosecutors said a drug dealer hired him to kill Wood because he did not want the judge to preside at his upcoming trial.
Charles Harrelson denied the killing, saying he was in Dallas, 270 miles away, at the time.
Wood, known as Maximum John for the sentences he gave in drug cases, was the first federal judge to be killed in the 20th century.
Charles Harrelson was transferred to Supermax, the highest-security federal prison, after attempting to break out of an Atlanta federal prison in 1995. Other inmates at Supermax, about 90 miles south of Denver, include Unabomber Theodore Kaczynski, Oklahoma City bombing coconspirator Terry Nichols and Olympic Park bomber Eric Rudolph.
His son got his start in acting as Woody the bartender on TV's Cheers beginning in 1985 and went on to star in films including Natural Born Killers,White Men Can't Jump and The People vs. Larry Flynt.
Woody Harrelson's publicist did not immediately return a call seeking comment Wednesday.
The actor was just 7 when his father was first sent to prison, for murdering a Texas businessman. He was in college when his father was convicted of the judge's assassination.

Harrelson has declared that he was involved in John F. Kennedy's assassination. Some think he was one of the three tramps photographed after being arrested on November 22, 1963, in a boxcar in the railyard near Dealey Plaza. Harrelson's arresting officer, Marvin L. Wise, claims that the three men in his custody were released after a few hours of questioning. The other arresting officer, David V. Harkness, testified that there were several individuals removed from the train that day other than the three individuals in the photograph. Dallas Police Department documents presented to the public in 1992 indicate that three transients arrested by Dallas officer W.E. Chambers with no connection to the assassination were jailed for six days for vagrancy, and that one of those men was named John Gedney.
Wikipedia biography of Woody's Dad!
Keenedy Assasination pics
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I bet none of you have/ had a dad like Woody's dad!!