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.