The Tri-State tornado of March 18, 1925, developed near Ellington, Mo., and then for the next 3.5 hours killed more people and destroyed more schools, homes and farmsteads than any other tornado to this point in history. The tornado cut across southern Illinois into southern Indiana, killing 695 people, 234 of them in the town of Murphysboro, Ill., and injuring 2,027. Other tornadoes occurred in Kentucky, Tennessee and Alabama. A total of nine tornadoes were reported, leaving 747 dead and nearly 2,300 people injured.
The Weather Service Office in Paducah, KY put together a great web page several years ago on the Tri-State tornado, which can be found below: