Animal rights organization PETA has released a press release urging HBO to investigate the death of a horse on the set of HBO’s The Gilded Age.

“PETA has heard from multiple whistleblowers alleging that a horse died last week during the filming of yet another HBO production,” the group wrote in a letter to Casey Bloys, HBO’s chief content officer. “Reportedly, the horse was 23 years old, had possible pre-existing health conditions, and the death was due to head trauma.”

“We’re calling on HBO to confirm the death, to conduct an immediate internal investigation into the incident, and to hold the party or parties who allowed it to occur responsible,” the organization wrote. “Finally, we’re asking you to take measures so that something similar never happens again.”

The network released a statement saying the horse died of natural causes according to a veterinarian’s findings.

“The safety and well-being of animals on all our productions is a top priority, and the producers of The Gilded Age work with American Humane to ensure full compliance with all safety precautions. Following AHA’s recommendation, the horse was transported to a facility for a full necropsy. AHA has interviewed all involved personnel, and full necropsy results are pending,” HBO said in a statement.

Back in 2012, three horses died on the set of the show Luck. “We had hoped HBO might have learned something from that experience: namely, that horses aren’t props,” PETA said in its letter.

“If it weren’t for these whistleblowers, the apparent death of this vulnerable horse would likely have been swept under the rug,” PETA senior vice president Kathy Guillermo said in a statement. “PETA is reminding HBO that watchful, compassionate people can be found on every set and is urging it to stop using horses before someone else dies in its productions.”

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