Kerry Condon on HBO’s “Luck.” ( Gusmano Cesaretti /HBO)

“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,” the premium cable network said in a statement late Wednesday.

Earlier in the week, People for the Ethical Treatment of Animals had asked the Los Angeles County District Attorney's Office and the Pasadena Humane Society to investigate the deaths of horses in the production of HBO's series "Luck," on which a third horse died this week.

Filming of scenes that involve the use of horses had been shut down indefinitely after the third horse died Tuesday. The horse was euthanized after rearing, falling backward and hitting her head on the ground while being led back to her stall. The horse had just been examined to make sure it was fit to appear in a scene for the second season of the drama series, which stars Dustin Hoffman as a crime kingpin trying to take control of a racetrack.

“While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won’t in the future,” HBO said in its 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,” the network said.

Mann and Milch also issued a statement with word of the cancellation:

“The two of us loved this series, loved the cast, crew and writers,” Mann and Milch said in a statement. “This has been a tremendous collaboration and one that we plan to continue in the future.”

The American Humane Association has been overseeing the program since it started production. The association's Los Angeles-based Film & TV Unit is the film and television industry's only officially sanctioned animal monitoring program. It's the organization that gives movies and TV series episodes that "No Animals Were Harmed" disclaimer you see at the end of programming.

PETA says the three dead horses were victims of “sloppy oversight.”

In its statement, the AHA pointed out that the most recent death did not occur on set, while filming or during racing, but rather while the horse was being walked back to its barn by a groom. And yet, AHA boasted: "We immediately demanded that all production involving horses shut down. We are also insisting that this stoppage remain in full effect pending a complete, thorough and comprehensive investigation."

In its complaint to Los Angeles law enforcement, PETA noted that the previous two “Luck” horses died - one in 2010, the other in 2011 - after breaking their legs during or immediately after running a second "race" in a day. Karen Rosa, senior vice president of the AHA's Film & TV Unit, told the TV Column that the "races" are short - no more than a a quarter-mile each.

PETA says that's still too taxing physically and mentally on retired racehorses, which “wouldn't understand that when they went through the starting gate on a racetrack, it was just for a TV show and not a real race.”

Neither of the first two horses that died during “Luck's” production “should have been anywhere near a racetrack," Kathy Guillermo, vice president of PETA, told The TV Column. The organization had claimed that one of the horses was so arthritic that it hadn't raced in years, and that the other horse was "so sore, he was given a potent cocktail of muscle relaxant and anti-inflammatory and painkilling drugs, including  Butorphanol, a painkiller so strong that it's often used as an analgesic for horses undergoing some kinds of surgery."

“Butorphanol is so powerful, they can castrate a horse on that drug,” Guillermo told the TV Column.

Rosa told the TV Column she didn't know where PETA could have got that information.

Guillermo, who's based in Northern California, handles issues having to do with racehorses for PETA. She told the TV Column that PETA got its information from the necropsy report and from “caring whistleblowers” concerned about the horses being used to shoot the series.

In response to PETA’s allegations, HBO had earlier in the day said in a statement that "Recent assertions of lax attitudes or negligence could not be further from the truth."

HBO's statement included a comment from the California Horse Racing Board director Rick Arthur, who said of this week's death: “Unfortunately, we see several of these injuries in the stable area every year. They are more common than people realize.”

AHA's "No Animals Were Harmed" disclaimer did not appear at the end of two episodes of "Luck" in its first season, Rosa told the TV Column.

In those two episodes, the language was modified, she said, and “indicated we monitored the animals. We did not say no animals were harmed.”