A series of horse deaths at Churchill Downs cast a shadow over the 149th running of the Kentucky Derby, which kicks off the prestigious Triple Crown thoroughbred racing series on Saturday.

Officials from the Louisville, Kentucky, racetrack said it was working with regulators to investigate after four horses died at the facility in the lead-up to the 1¼-mile race. One of those horses, Wild On Ice, was scheduled to run in the race.

On Thursday, Churchill Downs on handed an indefinite suspension to Saffie Joseph Jr., who trained two of the horses that died, Parents Pride and Chasing Artie.