Congress in December repealed a 34-year-old ban on the slaughter of wild horses that run free across the west.
The Humane Society of the United States says six mustangs were sold for slaughter on April 18 to Cavel International Incorporated in DeKalb.
An official at the U.S. Bureau of Land Management says it's investigating the details of the horse sale, which went to an Oklahoma man.
BLM spokeswoman Celia Boddington says the agency regrets what happened to the horses. She says it makes every effort to ensure the animals are placed in good homes when they're sold