The HBO series Luck, starring Dustin Hoffman, was canceled Wednesday after a third horse died on location.

It is with heartbreak that executive producers David Milch and Michael Mann together with HBO have decided to cease all future production on the series 'Luck,' HBO said in a statement.

Safety is always of paramount concern. We maintained the highest safety standards throughout production, higher in fact than any protocols existing in horseracing anywhere, with many fewer incidents than occur in racing or that befall horses normally in barns at night or pastures. While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won't in the future. Accordingly, we have reached this difficult decision.

A yet-unidentified horse reared and fell backward while being walked back to its stall after receiving a clean bill of health from a veterinarian. The horse hit its head and was later euthanized.

The American Humane Association's Film and Television unit has been monitoring the treatment of the animals used in production of the horse racing drama.

While this incident did not occur on set, while filming, or during racing, we immediately demanded that all production involving horses shut down, AHA Chief Communications Officer Mark Stubis wrote in an email statement on Wednesday.

HBO announced the cancellation several hours later. Given the tragic circumstances surrounding the death of a horse on Monday and in light of two other fatalities in 2010 and 2011, this is arguably the best decision HBO could have made, Stubis said in a follow-up statement.

In 2010, during filming of the pilot episode of Luck -- which included a dramatization of a downed racehourse -- a real horse was euthanized after suffering a serious fracture during a racing scene. A second horse was put down after sustaining an injury during the filming of the seventh episode.

At the time, People for the Ethical Treatment of Animals complained that efforts to communicate with show creator David Milch about safety protocols before filming began were rebuffed.

Now PETA is saying they tried to warn HBO about the likelihood of future injuries, and were again ignored.

Kathy Guillermo, PETA's vice president of laboratory investigations, said the group had briefly been in communication with HBO after learning of the first two deaths, and it was not satisfied with the revised safety protocols.

After learning from unidentified witnesses about the horse fatalities that took place during the filming of the first season, we wrote to HBO directly, Guillermo said in an interview. We received a response acknowledging yes two horses had died. ... And they went into some detail then about all the precautions that were put in place to prevent future injuries and deaths.

There were some things that were very disturbing to me about what was put into place to protect the horses ... and there were many unanswered questions.

PETA wrote back to HBO with a long list of follow-up questions.

At this point HBO wrote back and basically said 'at this point we've answered all your questions, there's nothing more to say. American Humane [Association] is monitoring it, we're fine.'

But I kept getting these messages from whistleblowers urging me to investigate further, Guillermo said. Once I put enough information together to indicate these horses should not be anywhere near a racetrack, our attorneys put together a letter to the [Los Angeles County] district attorney, to the Pasadena Humane Society, who has jurisdiction there, and asked for an investigation.

PETA's last direct communication with HBO was March 6.

What I kept hearing from whistleblowers was that horses were in danger on the set, there wasn't adequate oversight, and they feared another death. I wrote to HBO on March 6 to tell them what I was hearing and asked them to take action right away, Guillermo continued. And I did not hear back.

TMZ first broke the news of the latest horse tragedy, and later accused HBO publicist Karen Jones of lying about the initial reports and intentionally stalling the story.

HBO denied the allegations.

No one at HBO confirmed or denied any details regarding the accident that occurred on Tuesday until an official approved statement was issued, said HBO spokeswoman Nancy Lesser. This statement was provided to TMZ.com as soon as it became available.

No one stalled or lied about the story, she added.

But Gulliermo believes HBO would have kept the first two horse deaths to itself had PETA not first reported the news.

I don't think anybody was in any hurry to talk about it. I don't think it would have come out, Guillermo said, adding that she believed PETA alone was responsible for the story coming out.

I think there had been some scuttlebutt in the racing industry about it, but to my knowledge, when the New York Observer picked that up that was the very first time it was published by mainstream media, Guillermo said.

I don't believe that's what happened, said Jone Bouman, director of communications for the American Humane Association's Film and TV Unit. (Bouman forwarded the inquiry to a co-worker with closer knowledge of the situation, and IBTimes will update the story when we receive more information.)

Questions have surfaced in the past about the allegiances of the AHA - a 2001 Los Angeles Times investigation revealed that the group has been slow to criticize cases of animal mistreatment, yet quick to defend the big-budget studios it is supposed to police.

The article claimed that the unit lacks any meaningful enforcement power under the SAG contract, depends on major studios to pay for its operations and is rife with conflicts of interest.

When asked about funding from the Screen Actors Guild, Bouman insisted there was no conflict of interest.

We receive a grant from SAG and AFTRA because these animals are actors, Bouman said. We also need to be operating on donations just like another charity would.

We don't charge money when we go out on set, she continued. We don't make a salary from the studio or the production...we don't want to be beholden to anyone that would be able to say, 'You're fired.'

We're an independent, credible witness and we get authorization to go on the set because we're part of the contract.

But what the AHA's TV and Film unit actually has the power to do in situations where animal safety is threatened is a big concern for PETA.

I honestly don't know a lot about what their policies and protocols are, Guillermo said. What I heard that concerned me so much was who was on site during filming who was knowledgeable who could make sure the horses were safe...This is kind of a gaping hole in our knowledge at this point.

I'm not sure of anything, she continued. I want to know if there was somebody who was empowered to stop filming if an animal was in danger.

We will update this developing story once we have received further feedback from our sources.

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