What did rumors at the time say caused the Great Chicago Fire?

The Great Chicago Fire burned in the city of Chicago over three days in October, 1871. The fire killed approximately 300 people, destroyed roughly 3.3 square miles of the city, and left more than 100,000 residents homeless. City officials never determined the cause of the blaze. The most popular tale blames a cow, who allegedly knocked over a lantern in a barn.