Meanwhile, PETA blasts the network, series creators David Milch and Michael Mann and calls for filming on the horse racing drama to be shut down.

HBO has temporarily suspended the use of horses during production on Season 2 of its drama Luck after a third horse was injured on the set and euthanized.

The incident occurred early Tuesday when one of the horses in the stable for the Dustin Hoffman/Nick Nolte horse racing drama suffered an accident while returning to the stall.

Dr. Gary Beck, a veterinarian from the California Horse Racing Board, was on hand and noted the horse was on her way back to the stall when she "reared, flipped over backwards, and struck her head on the ground."

Attending vet Heidi Agnic administered immediate aid and it was determined that humane euthanasia was appropriate, Beck noted.

Tuesday's incident marked the third time a horse has been put down as part of the horse racing drama from David Milch and Michael Mann; two horses were injured and euthanized during production of Season 1.

HBO is working with the American Humane Association and the CHRB to investigate the incident, with a certified safety representative on the premises of the Santa Anita stable in Arcadia, Calif., and noted that all safety precautions were in place.

"We will not be filming horses until AHA completes its inquiry. Production will continue with other scenes," HBO said in a statement to The Hollywood Reporter. "Everyone associated with Luck cares deeply about the well-being of the horses who are so much a part of the heart and soul of the production."

HBO noted it works with the AHA and racing industry experts to implement safety protocols that go "above and beyond" typical film TV industry standards and practices, with pre-race exams performed by a CHRB-certified vet with radiographs taken of the legs of all horses being considered for use in simulated racing sequences.

"As with all fatalities within CHRB racing enclosures a necropsy will be conducted,"CHRB Equine Medical Director Dr. Rick Arthur said a statement. "Unfortunately, we see several of these injuries in the stable area every year. They are more common than people realize."

Meanwhile, People for the Ethical Treatment of Animals called for production on the series to be shut down and blasted the production for its use of what they claim are unfit or injured horses.

"HBO, David Milch and Michael Mann should be ashamed. Three horses have now died, and all the evidence we have gathered points to sloppy oversight, the use of unfit or injured horses and disregard for the treatment of racehorses," the animal-rights group said in a statement obtained late Tuesday by THR.

"We contacted HBO and producer/star Dustin Hoffman last week regarding reports we'd received stating that oversight on the set was lax, and we urged them to help make things safer for the horses. But we received no response. Yesterday, we went to law enforcement about the deaths of the first two horses, Outlaw Yodeler and Marc's Shadow, because one was drugged and the other was arthritic. We will want answers on HBO's latest casualty. Filming must stop now."

Despite its critical praise, Luck has been a ratings underperformer for the network, averaging about 625,000 total viewers. The series was renewed for a second season immediately after its lackluster premiere in order to fit production into the window between racing at Santa Anita.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit