Three horses died during the chuckwagon races at the 100th edition of the Calgary Stampede on Thursday.

Driven by Chad Harden, one of the lead horses went down in the backstretch, taking the rest with it and throwing Harden from the wagon. One horse was killed in the crash, while two others were euthanized afterwards. A fourth horse required surgery, but is expected to recover, according to reports. One of Harden’s outriders, Dustin Gorst, was also sent airborne when his horse hit the back of the rig (one of the horses who had to be destroyed). Both men were uninjured.

Critics of the event, who doubted improved safety measures, are shaking their heads and demanding an end to the dangerous sport. More than 50 horses have died in the chuckwagon races since 1986 as the result of collisions, heart attacks and missteps, for example.

The cause of the catastrophe is still under review. The other teams were able to avoid the disaster to finish the race.

UPDATE

Autopsy has revealed that a ruptured aortic aneurysm caused Harden’s left lead horse to collapse ands die, according to a statement released by the Calgary Stampede on Friday. Sadly, even the event’s improved veterinary protocols could not detect such a pre-existing condition. Toxicology and drug tests, however, will also be conducted to determine if any additional factors contributed to the death.