NEOSHO, Mo. (AP) — Authorities say 53 puppies died of overheating when they were left in a vehicle in southwestern Missouri.

The Newton County Sheriff’s Department said a truck company employee was struggling to keep the puppies warm Sunday night because temperatures outside were in the single digits and the truck’s heater was malfunctioning. Chief deputy Chris Jennings said the worker added an extra heating component and found the dogs dead after returning to check on them 90 minutes later. Jennings described it as a “horrible accident.”

Renee Ray, an owner of the trucking company RDR Transport, says some puppies survived and were treated by veterinarians.

The sheriff’s department doesn’t plan to seek charges, but the U.S. Department of Agriculture is looking into the deaths to determine if animal welfare laws were broken.