Credit: Gusmano Cesaretti/HBO/AP Photo

The HBO Series "Luck" will cease production after three horses died during filming, the network and the series' executive producers said today.

"Safety is always of paramount concern," HBO said in a press statement. "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."

Executive producers Michael Mann and David Milch joined in: "The two of us loved this series, loved the cast, crew and writers. This has been a tremendous collaboration and one that we plan to continue in the future."

Production had already been suspended twice while the animals' deaths were investigated.

The American Humane Association, which oversees Hollywood productions involving animals, had demanded that production shut down. The association called for a "complete, thorough, and comprehensive investigation" into the deaths.

The most recent case involved a horse that had just been inspected by Dr. Gary Beck of the California Horse Racing Board and was being walked back to the stable at Santa Anita Racetrack, in Arcadia, Calif., where much of the filming takes place.

"The horse was on her way back to the stall when she reared, flipped over backwards, and struck her head on the ground," Beck said in a statement yesterday.

Beck and a second veterinarian, Dr. Heidi Agnic, said they decided euthanasia was the humane way to deal with the horse's injuries.

Actor John Ortiz, who played horse trainer and gambler Turo Escalante on the show, tweeted in response to the accident, saying, "Everyone on set loves and cares for the horses. Their well-being is the top priority in the process."

The series was in its second season. During the filming of its first season, in 2010 and 2011, two horses were injured and euthanized. Filming had resumed in February after new protocols were put in place.

According to autopsy results, the first horse was a 5-year-old male who suffered from a "catastrophic humeral fracture" of its right front leg and was on multiple pain killers and anti-inflammatory medications. The second, an 8-year-old male, had an open fracture of its right radius and a history of arthritis.

People for the Ethical Treatment of Animals criticized HBO after the deaths of the horses. The group said the show used older race horses during filming and asked them to do more than a young thoroughbred would have to.

"The horses don't know it's not a real race," said Kathy Guillermo, PETA's vice president of laboratory investigations. "Racing is an exhausting activity and most young horses have at least a week's rest between races to recover. These horses were sometimes asked to race twice in a day."

"Luck," starring Dustin Hoffman, told the story of a crime kingpin trying to gain control of a race track and launch casino gambling.