FRESNO, Calif. (AP) — An Agriculture Department spokesman says officials don't yet know what causes the strain of mad cow disease that struck a California dairy cow.Federal officials say the cow was very old for a milk producer and had been euthanized after it became lame and started lying down.The 10-year-old dairy cow from Tulare County, only the fourth ever discovered in the United States, was found as part of an Agriculture Department program that tests about 40,000 cows a year for the fatal brain disease.U.S. health officials say there is no risk to the food supply. The cow was never destined for the meat market, and it developed "atypical" bovine spongiform encephalopathy from a random mutation, something that scientists know happens occasionally.Agriculture officials are investigating, among other things, whether feed sources might have played a role in the animal contracting the fatal illness.