When the rancher showed up, he realized they died of thirst. At least seven cows died after the cattles' water supply was cut off by an apparent trespasser.
Contra Costa Sheriff's Animal Services is investigating.
"Someone had turned the automatic valve off and therefore, drying up the trough, the only water source for about 50 head of cattle that he had out there," said Lt. Abe Gamez.
Neighbors first contacted rancher Art Anderson when his cows began having problems.
"The cows have really been acting strangely this last week," neighbor Bev Marshall told ABC7. "Watering my backyard, they're staring at me and so I'd water them down, and they'd hold their mouths open for water. So, you knew something was wrong. "
"It was very hot. It was over a 100 on Saturday and they were dying," said another neighbor Kathy Barr.
Anderson said he is trying to gradually rehydrate his remaining herd to keep more animals from dying on the land he leases from the Navy. He recently moved his cattle from their usual grazing area to keep them safe during fire training exercises held at the weapons station last week.
"He had to have them in that area and the gates closed, so they couldn't enter the other areas," explained Lt. Gamez.
Neighbors say they try to keep an eye on all the wildlife that live on the vast federal property.
"Everyone else that can is cooling off and these animals are dying right in the backyards here," pointed out neighbor Kathy Gleason. "I'd really like to know how it happened. I'd really like to know who's responsible for this."
The weapons station has had its share of problems with trespassers going onto the property to steal copper and other metals. It is not clear in this case if the person who shut off the water valve meant to harm the cattle. But, if it was done intentionally they could face criminal charges.