HBO has agreed to stop filming with horses on Dustin Hoffman drama Luck after a third animal was injured and euthanised during production. The American Humane Association (AHA) issued the order, pending a \"thorough and comprehensive investigation\". HBO said the horse was being led to a stable by a groom when it reared and fell back, suffering a head injury. The animal was euthanised at the track in suburban Arcadia, California, where Luck is filming its second series. Although the AHA - which oversees Hollywood productions - noted the accident did not occur during filming or racing, it issued the demand \"that all production involving horses shut down.\" On Tuesday, California Horse Racing Board vet Dr Gary Beck said he had just examined the horse as part of routine health and safety procedures before it was to race later in the day. \"The horse was on her way back to the stall when she reared, flipped over backwards, and struck her head on the ground,\" Dr Beck said in a statement. A second vet determined that euthanasia was appropriate, he added. \'Deeply saddened\' Dr Rick Arthur, medical director of the state racing board, said such injuries occur in stable areas every year and are more common than thought. HBO said in a statement that an AHA safety representative was at the track when the accident occurred and \"as always, all safety precautions were in place\". It added it was \"deeply saddened\" by the horse\'s death and was working with the AHA on its inquiry. During filming of the first series in 2010 and 2011, two horses were hurt during racing scenes and were subsequently euthanised. The AHA called for a production halt at the Santa Anita Racetrack after the second horse\'s death, and racing resumed in February after new protocols were put in place and had proved effective. The first two horse deaths drew criticism from animal rights group Peta, which said the safety guidelines were \"clearly inadequate\" as they failed to prevent the deaths. On Tuesday, Peta vice-president Kathy Guillermo said: \"Three horses have now died and all the evidence we have gathered points to sloppy oversight, the use of unfit, injured horses, and disregard for the treatment of thoroughbreds.\" The series, which looks at racing\'s seedier side, sees Oscar-winner Dustin Hoffman play a crime kingpin scheming to gain control of a racetrack and introduce casino gambling. It is currently being shown in the UK on the Sky Atlantic channel.