ARNOLD, Mo. (AP) — A Missouri police dog died when the air conditioner failed in the patrol vehicle he had been left in, police said.

Vader, a 4-year-old K-9 for the Arnold Police Department in suburban St. Louis, died Wednesday. Temperatures this week have been in the 90s, with high humidity.

A Facebook posting from Arnold police said Vader's handler left the dog in the running, air-conditioned vehicle while the officer tended to other duties, a practice that police said “is necessary and common practice when the K-9 partner is not actively engaged in police work.”

But when the handler returned to the vehicle, it was discovered that the air conditioning system malfunctioned, police said. The dog was taken to a veterinarian clinic and initially showed signs of improvement but later died.

Police said the department's vehicles for use with police dogs include a system that notifies the handler by phone, activates a cooling fan, even rolls down the windows if the temperature reaches a certain level. “In this instance, the heat alarm system failed to activate,” police said.

It was the second hot-car death involving a Missouri police dog this year. In June, a dog named Horus died in a hot car in Savannah, Missouri.