According to the following passage, what was the cause of the Great Chicago Fire?

The fire is claimed to have started at about 8:30 p.m. on October 8, in or around a small barn belonging to the O'Leary family that bordered the alley behind 137 W. DeKoven Street. The shed next to the barn was the first building to be consumed by the fire. City officials never determined the cause of the blaze, but the rapid spread of the fire due to a long drought in that year's summer, strong winds from the southwest, and the rapid destruction of the water pumping system, explain the extensive damage of the mainly wooden city structures. There has been much speculation over the years on a single start to the fire. The most popular tale blames Mrs. O'Leary's cow, who allegedly knocked over a lantern; others state that a group of men were gambling inside the barn and knocked over a lantern. Still other speculation suggests that the blaze was related to other fires in the Midwest that day.

There has been much speculation to the cause of the Great Chicago fire, but city officials never determined the cause of the blaze. The most popular theory is that it was caused by Mrs. O'Leary's cow, who knocked over a lantern.