Police in Kentucky are hunting whoever is responsible for killing 20 horses in an "inhumane and cruel" act that created a "battlefield for horses".
The Dumas Rescue group said some of the horses were pregnant mares, others foals up to one year old. It is offering a $20,000 (£15,460) reward.
The herd of about 35 horses roams over a wide area of eastern Kentucky, making the search for survivors difficult.
The horses had been shot with a low-calibre weapon, police said.
The bodies have been found spread out across the site, an open-cast mining area, and Dumas Rescue is covering about 4,000 acres (1,620 hectares) a day.
Killings were first reported on 16 December, with the latest six dead animals found on Sunday.
Floyd County Sheriff John Hunt said this was one of the worst cases of animal cruelty he had ever seen.
He told WYMT: "This is very inhumane and it's a very cruel act of somebody who just apparently had nothing else to do."
According to the sheriff, one of the horses still had grass in its mouth when it was shot dead.
The herd, which Dumas says is largely made up of animals abandoned by owners, is very approachable and the horses are often fed by members of the local community.
It is thought the horses were all killed at the same time. According to the rescue group, the animals appear to have been hunted.
Dumas Rescue is now trying to find a way of getting the remaining horses out of the area. However, it expects this to be difficult as the animals will be stressed following the shootings.