DENVER -- Actor Woody Harrelson's father, Charles Voyde Harrelson, died of a heart attack in the Supermax federal prison where he was serving two life sentences for the murder of a federal judge, officials said Wednesday.

Charles Harrelson, 69, was found unresponsive in his cell 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 suddenly in his sleep at about 10 p.m. the night before.

"He just looked like he was comfortably asleep," said Twellman, who is also an emergency room physician who was on duty when Harrelson's body was taken to St. Thomas More Hospital in Canon City, about six miles from the prison. "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 "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 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.

Charles Harrelson was named in the book "The Man On The Grassy Knoll" as one of the gunmen behind the picket fence at Dealy Plaza in Dallas, when President John F. Kennedy was assassinated. National columnist Jack Anderson also believed Harrelson was involved.

Harrelson told a television producer that he was in Houston when Kennedy was shot.

Woody Harrelson's publicist did not immediately return a call.

Read an online discussion on Harrelson's death.