According to the Globe and Mail, Dr. Ancel Keys, who was hired by the US War Department in 1941 to develop "pocket-sized food rations that paratroopers could carry in combat", died on November 20, aged 100, in Minneapolis.
According to the obituary, "when the U.S. army mass-produced the packages, he was surprised to see them marked with the letter K, for Keys." I don't know if that's really the origin of the K in K-rations, but I guess the fact that he was contracted by the War Department is a matter of public record.

He was also instrumental in establishing a link between saturated fats and heart attacks, and was featured on the front cover of Time magazine in 1961, in an issue devoted to diet and health.