State agriculture inspectors can’t say what killed 50 to 60 cows last year on a dairy farm near Addy, Wash., but they can say what didn’t.

It wasn’t any contaminants in the feed the cows ate. It wasn’t lead or arsenic or heavy metals in the hay. It wasn’t mad cow disease, or any other communicable illness.

State Veterinarian Leonard Eldridge said an investigation found “no threat to the health of people or other animals.” The investigation turned up no common cause for the deaths of the cows, which started last May and continued for about seven months.

“Frankly, we may never know specifically what killed the animals that died before the start of this investigation,” Eldridge said in a news release issued Monday.

The dairy farmer voluntarily stopped shipping milk from his operation in December. Investigators visited the farm in March after reports that the cows had died over a short period of time.

Initial reports were “very sketchy,” Jim Matsuyama, environmental health director for the Tri-County Health District, said.

The farmer, whose name was not released, told state Department of Agriculture inspectors that he suspected heavy metals had contaminated the feed sources, but the state Disease Diagnostic Lab found no unusual levels of chemicals in the feed. Investigators then tested three animals from the herd, which were killed and checked for heavy metals as well as Bovine Spongiform Encephalopathy, commonly known as mad cow disease, and other potentially fatal diseases. Those tests were also negative.

Eldridge suggested that the farm improve “the general sanitary conditions” and get advice on health management and nutrition for the herd.

Jason Kelly, a spokesman for the state Agriculture Department, said if the dairy wants to resume its milk shipments it will need to pass an inspection, which would review sanitary conditions.