The owner arrived at the field and was trying to hydrate the surviving horse, Hansen said.
Kennecott, which owns the land, leases it to Erda Livestock, said Kennecott spokesman Kyle Bennett.
Dennis Morgan, whose back fence borders the field, normally sees the horses running about for one to two months a year before the owners move them elsewhere. But lately, he hadn't seen them for a couple weeks, and when he started to smell "something foul" and saw vultures descend into the field, he went up to the cemetery to find out what happened.
That's where he saw two of the horses topple and fall, he said.
"To see this, it's horrible," said Morgan, who called for help. "It's horrible to allow these beautiful animals to die like this."
The horses come and go from the property, with enough time in the interim for the grass to grow back, Morgan said. A water tank normally arrives to refill the horse's trough, but Morgan could not recall seeing the delivery this year.
Besides a trough, the field has a spigot but no water was coming out of it, Hansen said.
About 8 p.m., Unified Police returned to the field with Sheriff Jim Winder and the mounted posse to continue their search for more horses, as well as document the water resources. Based on what a witness told them, officers believe there might be a few more horses in the area.
High temperatures reached 96 degrees Friday, according to the National Weather Service.