Edgar Ray Killen, a former Ku Klux Klan leader who was convicted for the 1964 “Mississippi Burning” killings of three civil rights workers, has died in prison at the age of 92.

Killen was serving three consecutive 20-year terms for manslaughter when he died at 9pm on Thursday inside the Mississippi State Penitentiary at Parchman. An autopsy was pending, but no foul play was suspected, a statement from the state’s corrections department said.

Killen’s conviction came 41 years after James Chaney, Michael Schwerner and Andrew Goodman, all in their 20s, were ambushed and killed by Klansmen.

The three “Freedom Summer” workers had been investigating the burning of a black church near Philadelphia, Mississippi. A deputy sheriff in Philadelphia had arrested them on a traffic charge, then released them after alerting a mob.

Mississippi’s then-governor claimed their disappearance was a hoax, and segregationist Senator Jim Eastland told President Lyndon Johnson it was a “publicity stunt” before their bodies were dug up.

The killings shocked the nation, paved the way for the landmark civil rights act of 1964 and were dramatised in the 1988 movie Mississippi Burning.

Killen, a part-time preacher and lumber mill operator, was 80 when a Neshoba County jury of nine white people and three black people convicted him of three counts of manslaughter on 21 June 2005.

Killen was the only person to face state murder charges.