PLEASANT PRAIRIE, Wis. (WTMJ/CNN) - It's the worst case of horse abuse Wisconsin investigators have ever seen, and it could get even worse.

Police found at least 60 dead horses in a mass grave behind a farm in Pleasant Prairie, Wis. Now they think there could be even more.

Officials suspect some of the horses died within the past six months. They're currently testing remains to determine the cause of death.

Deputy Police Chief David Morgensen says officials are still searching the property.

"We don't know why there's that many carcasses buried back there. Was that a business? Was that just animals that died off? Was there criminal conduct that caused them to die? That's what we need to find out," said Morgensen.

When investigators first executed search warrants on the property they found five horses dead in their stalls. Tests showed starvation contributed to their deaths.

The owners, Paula and David White, were arrested and charged with animal mistreatment.