RAPID CITY, S.D. -

The cause of a prairie fire that burned 64 square miles of grassland and killed nearly 300 cattle in western South Dakota last month might remain a mystery.

The Cottonwood Fire east of Rapid City scorched private land, state land and part of the Buffalo Gap National Grassland. It destroyed numerous rural buildings. No one was hurt.

Authorities say the fire was caused by a human, but that the precise cause might never be known. U.S. Forest Service District Ranger Kurt Tindel says officials don't think the fire was intentionally set.

Adele Harty with the South Dakota State University Extension Service says many of the dead cattle burned to death, while others survived but later had to be euthanized because of injuries.