The warning came from someone identified as "The General."
A handwritten note addressed to Sanders and his wife told the restaurant entrepreneur from Kentucky that he was in "grave danger of being murdered."
The documents show that the envelope containing the threat was postmarked Dec. 5, 1973, and had a return address in Los Angeles. The threat wasn't reported to the FBI until February 1974 by someone at KFC whose name was blacked out.
Sanders died in 1980 at the age of 90 after being stricken with leukemia.