Luck production chiefs rescinded its American Humane Association stamp of approval - which certifies no animals were harmed during the making of the programme - following the show's pilot episode after a horse was euthanized on location.
Prior to filming, network executives at HBO assured officials at People for the Ethical Treatment of Animals (PETA) that experts would be on hand to ensure all "necessary safety procedures" were in place, however reports of a second fatality have again prompted activists to worry.
PETA Vice President Kathy Guillermo tells Entertainment Weekly, "We asked for the names of the horses, whether they were retired racing horses and what their records had been, what physical condition they were in, what their rest periods were, and if they were checked between racing sequences.
"When we began to ask uncomfortable questions, they closed the door on us. We received an email this morning saying all this information is confidential and that they’re doing all that they can to prevent injuries."
American Humane Society bosses conducted a full investigation with members of the California Horse Racing Board, which has jurisdiction over the Santa Anita racetrack, where there show is filmed.
However, PETA officials have requested a review of the official necropsy reports and they're now pushing for harsher safety guidelines when it comes to horses used on TV and movie sets.
Guillermo adds, "Breakdowns don’t just happen. They happen every day, obviously, but they don’t happen in the absence of conditions that create them. Horses breakdown for a reason, and often it has to do with the condition they’re in at the time they’re put on the track. So we want to know who were these horses that died."
One death occurred during the making of the pilot and the second fatality occurred while filming episode seven.
Otherwise the new show is already a big hit and has been picked up for a second season.