Fire officials in Pennsylvania are investigating the cause of a barn fire responsible for the death of more than 20 horses on Sunday.

Chinchilla Fire Chief Keith Grierson said the blaze involving a 50-foot-by-300-foot barn at the English boarding and training facility Over The Hill Farm in Clarks Summit near Scranton, Pa., was reported at about 6 p.m. on July 24. A total of 29 horses were located inside the barn at the time of the fire.

"Of those, 21 horses perished," Grierson said.

Six horses are being treated for fire-related injuries, and two other animals remain unaccounted for, Grierson said. Two firefighters working the blaze were taken to a nearby hospital where they were treated for heat exhaustion and released, he added