The Tri-State tornado was first sighted in Shannon County, Missouri, about 12:40 p.m. on March 18, 1925, and the first fatality occurred about twenty minutes later north-northwest of Ellington. The storm virtually annihilated the town of Annapolis in Iron County, killing two people there. Two more people were killed at the small community of Leadanna, also in Iron County. The twister then crossed Bollinger County and entered Perry County, where it struck the town of Biehle, destroying many homes and killing four people. At least eleven people were killed in Missouri before the tornado continued its deadly path through Illinois and into Indiana.
The storm cut a swath 235 miles long and about 1,200 yards wide on average. Altogether it killed 695 people, making it by far the deadliest single tornado in U.S. history. Although not officially rated at the time, it is considered an EF5 tornado.
The Tri State tornado was part of a whole series of tornadoes that broke out on the same day, and the total number of deaths for all the storms that day was 747.