Nine cows died after drinking from Delmoe Lake, a popular recreational lake east of Butte.

A witness reported watching two of the cows drink water from the lake, then walk just a few feet before falling over dead. This has prompted a warning for people to stay out of Delmoe Lake.

Terry Sexton is Whitehall district ranger for the Beaverhead-Deerlodge National Forest. He says the cause may have been a blue-green algae bloom.

Forest Service officials are posting signs warning people to stay away from the lake.

Dr. Joe Hartley is a large-animal veterinarian in Dillon. He says blue-green algae, when in the bloom growth phase, can produce a cyano-toxin that is incredibly lethal.

The suspected algae are killed by a hard frost, so officials are hoping to get a cold night to take care of the problem.