No one has been charged, and police were continuing their investigation.
The fire raged through a barn that housed 120 horses at the racetrack in the early morning hours. Total damage was estimated at $3.2 million.
Earl Barnett, who lost five of his 16 horses in the fire, called the arson ruling ``a complete shock.''
``I was under the impression it was a cigarette cause or electrical, but deliberate arson _ that's our worse-case scenario thinking,'' he said.
Another horse owner, Steve Owens, said he always suspected arson because the fire started in a part of the barn that lacked video surveillance cameras. He lost 14 horses in the blaze.
Bill Hiscott, acting supervisor at the fire marshal's office, recommended steps to improve fire safety at racetracks, including the installation of sprinklers in stables, and storing hay and other combustible material elsewhere.
Hugh Mitchell, senior vice president of racing at Woodbine, said the racetrack has hired a fire safety officer and is installing sprinklers in barns while upgrading electrical wiring and taking other steps.
