People for the Ethical Treatment of Animals (PETA) is outraged after a horse died during the production of season 2 of HBO’s Gilded Age. The animal rights activists are now calling on the network to investigate the death.

“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,” PETA’s Courtney Penley wrote in a letter to HBO Chief Content Officer Casey Bloys, according to a report by Variety.

“Finally, we’re asking you to take measures so that something similar never happens again,” Penley added.

The PETA letter also cited a whistleblower who said a 23-year-old horse with possible health issues died while filming Season 2 of the series.

After the letter was shared with the media, HBO confirmed that the horse had died on set June 28 during filming, adding that its death was “likely of natural causes, according to a veterinarian’s preliminary 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,” HBO said.

The network added that the horse was transported to a facility for a full necropsy, and that the American Humane Association has interviewed everyone who was involved in the matter.

Horse wranglers and a veterinarian said the horse was not showing any discomfort from the temperature — which was in the 70’s — and had not been overworked or overheated before it collapsed, sources told Variety.

They added that the scene that was being shot when the horse died involved a carriage and two passengers.

Nonetheless, the PETA letter also mentioned another HBO racing drama, Luck, that was canceled in 2012 — after one season — following reports that three horses had died during production.

“We had hoped HBO might have learned something from that experience: namely, that horses aren’t props,” Penley wrote. “They’re sensitive animals who can be startled easily, and they must be gradually accustomed to the changing conditions on a set.”

The PETA spokesperson argued that horses should not be used in film or television, but that if they are, an equine behaviorist should be on set at all times.

In 2016, PETA blasted HBO over an episode in the series, Silicon Valley — which they called “horse porn” — arguing that the practice of horse breeding is “arranged rape.”

An episode of the popular HBO show had gone viral online after two characters were distracted during a conversation about their startup company by two horses having graphic sex.

You can follow Alana Mastrangelo on Facebook and Twitter at @ARmastrangelo, and on Instagram.