SARDIS, Miss. — Eleven of 50 dogs being transported by an animal rescue group from Hattiesburg to Minnesota died when the van carrying them broke down on Interstate 55 in north Mississippi.

The van had left the Southern Pines Animal Shelter about 1:30 p.m. Friday before being forced to stop near Sardis, about four hours into the trip.

Two volunteer drivers discovered the dead dogs as they worked to unload them, the Clarion Ledger reported.

A statement from the shelter on its Facebook page Saturday said the vehicle was undergoing a thorough inspection to try to determine the cause of the deaths.

“Upon leaving the shelter all the dogs were alert and healthy, the van’s AC systems and ventilation seemed to be in working order, and throughout the trip the van remained cool,” the shelter said. “We are working to investigate what other system failures or mechanical malfunctions could have impacted the animals on board, and we are hoping to find some definitive answers as to what happened.”

Group Director Ginny Simms tells WREG-TV that they believe carbon monoxide poisoning from a mechanical malfunction — not the heat — may have been to blame for the accident.

Sims said the remaining animals remain under observation even though none are exhibiting signs of any heat-related issues.

“We are committed to helping save the lives of pets in need, and any life lost in our care is something we take very seriously,” the organization’s post on Facebook said.

“To say that we are grieving and devastated by the terrible accident that occurred yesterday would be an understatement.”

The shelter also thanked “all the compassionate passersby and to North Mississippi Animal Rescue for their help” in the aftermath of the accident.