SIMSBURY, Conn. (AP) — Investigators say a barn fire that killed 24 horses at a Connecticut equestrian training and boarding farm started accidentally.

Simsbury fire marshal Kevin Kowalski says Friday the blaze likely started in an electrical outlet that was used for portable heaters in the barn. He says it appears the fire was smoldering for several hours before it was spotted and fire crews were called.

The fire occurred Thursday morning at Folly Farm, a 175-acre, family-owned farm about 10 miles (16 kilometers) southwest of Hartford.

The horses died of smoke inhalation.

The barn, which was heavily damaged, was the largest one on the farm. Nineteen other horses were housed in buildings that were not damaged.