HBO's new series Luck, has been cancelled unexpectedly after growing safety concerns for its equine cast members boiled over. The show starred Dustin Hoffman and Nick Nolte. Three horses have died in its short production span.

On Tuesday, a horse was injured and euthanized at Santa Anita Park racetrack, according to USA Today, and HBO agreed to suspend filming at the request of the American Humane Association, a group that oversees Hollywood productions.

HBO's Luck had seen the death of two horses during shooting prior to Tuesday's incident, both of which generated outrage from the People for the Ethical Treatment of Animals (PETA).  PETA sent a letter to L.A. District Attorney Cooley one day prior to the death of the third horse.

I am writing on behalf of PETA and its more than three million members and supporters, including more than 200,000 of whom reside in California, to respectfully request that you investigate and take appropriate action against those responsible for causing two seemingly unfit horses to be overdriven and inflicted with unnecessary pain, resulting in death, in violation of Cal. Penal Code § 597, Lindsay Waskey, a PETA counsel, said in the memo.

The horses were forced to engage in racing sequences for the HBO televisions series 'Luck' at the Santa Anita Race Track, which tragically resulted in the fatal and catastrophic breakdowns of both horses, Waskey said.

Ratings for Luck were weaker than other high-cost productions from HBO. The company said Wednesday that Luck averaged 4.8 million viewers cumulatively across airings of its episodes reports the Wall Street Journal.

Gary Stevens, a retired hall-of-fame jockey that starred in the show, spoke publicly about the cancellation in a series of tweets.

So bummed. Peace out to all my family in #luck, tweeted Stevens. He followed up with several more tweets, retweeting messages from fans and answering questions from others.

John Ortiz, another actor from the series, tweeted, Ain't gonna lie...Today was tough, rough and kinda dark! Thank you all for helping me get through it. Ever upward! #Luck

HBO maintains that it's very proud of the cast and crew for what they were able to accomplish: While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won't in the future, the channel said in a statement. Accordingly, we have reached this difficult decision. We are immensely proud of this series, the writing, the acting, the filmmaking, the celebration of the culture of horses, and everyone involved in its creation.