Mike McMillan/AP

In this photo provided by the U.S. Forest Service, a Hotshot fire crew member rests near a controlled burn operation at Horseshoe Meadows, as crews continue to fight the Rim Fire near Yosemite National Park in California Wednesday, Sept. 4, 2013.

It's been four weeks since the Rim Fire began burning near Yosemite. So far, it's consumed nearly 400 square miles, making it the third largest fire in California history.

A hunter's illegal campfire allegedly started the blaze, but how did investigators figure that out?

For answers, we reached out to Paul Steensland, a retired fire investigations specialist who spent more than 20 years with the U.S. Forest Service.