Just before 7 a.m. Sunday, Debbie Stephens, a renowned American equestrian, arrived at her horse barn in Palmetto, Fla., to check on paperwork and prepare for the day’s lessons.

Suddenly, one of the barn workers appeared with unsettling news: The latch on the stall of a prized Grand-Prix-level show jumper had been tampered with, and the horse was nowhere to be found.

Within a half-hour, there were signs of an unusual crime, then a brutal discovery: The horse had been led from its stall and taken far from the barn, where it was carved up so professionally that the authorities are investigating it as a case of animal cruelty carried out by an expert butcher for meat.

“He had been filleted,” Ms. Stephens, the horse’s owner, said in a telephone interview on Monday. “The slices were so deliberate and so well done that the moment you saw it: This was a professional.”