06/11/2013 04:27 pm ET Updated Jun 11, 2013

James King, Key Figure In Denver Father's Day Massacre, Dies

The lone figure to be tried and acquitted in Denver's still-unsolved Father's Day Massacre passed away almost 22 years to the day of the crime.

James King was arrested and charged with four counts of murder for the June 16, 1991 Denver bank heist that left four guards dead at the then-United Bank of Denver on 17th Avenue and Lincoln Street. The gunman had killed Phillip Lee Mankoff, Scott Raymond McCarthy, William Rogers McCullum Jr. and Todd Allen Wilson all execution style.

King's son told The Denver Post that his father died May 21st of dementia at the Hospice of Saint John in Lakewood. He was 77 years old.

Predating the Columbine High School massacre by about eight years, it was Colorado's largest mass killing.

The gunman made off with nearly $200,000 that day that has never been found. He retrieved all the bullet casing, wiped away any fingerprints and took the videotapes from the bank's surveillance cameras.

It didn't take long however for the case to fixate on King, then 54 years old.

Clues led Denver homicide detectives to believe the heist was an inside job and may have been committed by a policeman.

King was a retired sergeant from the Denver Police force and had worked as a weekend security officer at the bank a little less than a year earlier. He also didn't have a strong alibi for the morning of the heist.

"I believe he did it," Former Denver Prosecutor Bill Buckley told The Denver Post. "Unless you hear he confided in someone, he took his secret to the grave."

The trial gained national attention and was broadcast on Court TV in 1992. After nine days of deliberation however, the jury acquitted King. No one else has been arrested or charged with the crime since.