The South Texas Trail Riders said the two horses may have been poisoned during a ride last week.
The group posted on its social media accounts warning trail riders to stay cautious if they were parked at the Martinez Hall around Feb. 9.
According to the group, one of the horses was found dead while the other had to be put down. The post further stated a veterinarian is working to figure out what type of poison was used to kill the animals.
The trail riders added law enforcement is looking for the people involved and they are asking other participants to keep a close eye on their horses.