Simone Netherlands, leader of the Salt River Wild Horse Management Group, has been leading the effort to find and identify the horse's remains.
She said in addition to the 25 confirmed dead, there are 25 more that are missing.
"We logged them as missing because we would see one horse dead out of that group," she said.
Netherlands said she doesn't expect to find the missing horses alive, which would make 50 horses shot and killed.
Because of a long-standing argument over whether these horses are "wild" or feral, horse advocates have logged 350 out of the approximately 400 horses in that area, Netherlands said, which is how they've identified the 25 that went missing.
“We have to fight for these horses, Netherlands said, "and it's literally a battle for their protection so that we can have this piece of very important Arizona history left.”
The horses have all been shot, Netherlands said. Many have been shot in the body or neck and between the eyes.
The reward for information that leads to a conviction has also been raised to $35,000.
Up to Speed
Catch up on the latest news and stories on the 12News YouTube channel. Subscribe today.
More ways to get 12News
On your phone: Download the 12News app for the latest local breaking news straight to your phone.
On your streaming device: Download 12News+ to your streaming device
The free 12News+ app from 12News lets users stream live events — including daily newscasts like "Today in AZ" and "12 News" and our daily lifestyle program, "Arizona Midday"—on Roku and Amazon Fire TV.
12News+ showcases live video throughout the day for breaking news, local news, weather and even an occasional moment of Zen showcasing breathtaking sights from across Arizona.