The Ku Klux Klansman convicted of the notorious Freedom Summer killings of three civil rights workers in Mississippi in 1964, has died in prison.

Edgar Ray Killen, who would have turned 93 next week, died on Thursday night at the Mississippi State Penitentiary's hospital, authorities said.

Edgar Ray Killen sits in court in 2005. Credit:Rogelio V. Solis

His cause of death would be established in an autopsy but no foul play was suspected, they added.

The former Klansman was serving a 60-year sentence for manslaughter over the deaths of James Chaney, Andrew Goodman, and Michael Schwerner, who had been trying to sign up black voters when they were killed.