Three horses are dead and one is injured following a chuckwagon crash during a Thursday night race at the Calgary Stampede, officials confirm.

This story is from 2012. Two horses have died so far in the 2015 Calgary Stampede. Read about the latest Stampede horse death here.

In a video of the accident, Chad Harden's wagon takes a serious tumble in the back stretch of the Rangeland Derby's fourth heat.

The tumble brought down a number of his horses and an outrider — a cowboy who loads poles at the start and must finish the race with the chuckwagon.

Stampede spokesman Doug Fraser said two horses had to be put down by veterinarians. Harden and the outrider are both fine.

"One of Harden's outrider horses also collided with the wagon and sustained serious injuries," Fraser told reporters in a prepared statement. "The left lead horse, which was the horse that collapsed, experienced a fatal event the nature of which is undetermined at this time. The right lead horse and the outrider horse were humanely euthanized due to the severity of their injuries."

The horse on the right wheel sustained non-life-threatening injuries and is receiving veterinary attention, according to officials. The injuries will require surgery, but the horse will survive.

The left wheeler and the other outrider horse were not injured.

Chuckwagon driver Chad Harden speaks to reporters shortly after a crash during a Thursday night race led to the deaths of three of his horses. (CBC)

A tearful Harden, overcome with grief after the loss of his horses, spoke to reporters shortly after the animals were euthanized on the dirt track, calling them "part of our family."

"We love those horses," he said, his voice breaking with emotion. "The outriding horse is an 18-year-old horse, I've had him for 13 years. He's part of our family, he's supposed to be my kid's horse."

Speaking of his right lead horse, which helped Harden win the 2009 Calgary Stampede, he said the horse was irreplaceable.

"They're just like humans, they're our family," he said. "It's just devastating for our whole family. It's hard to take."

P.O.V. Should the Calgary Stampede's chuckwagon race be banned? Take our poll.

The Stampede introduced changes last year aimed at making chuckwagon races safer for both horses and competitors.

Vancouver Humane Society calls for ban

The move came after six horses died in 2010, two from heart attacks.

All horses are now inspected by veterinarians when the animals arrive at the Stampede, and before and after every race.

What is chuckwagon racing? The sport, at the Calgary Stampede since 1923, represents pioneer cowboys breaking up camp and racing their chuckwagon rigs back to town after working out on the range. A driver commands a team of thoroughbred horses supported by two outriders who follow the chuckwagon and must finish the race with it. A race starts with the outriders "breaking camp," mounting their horses and following the wagons as they complete a Figure 8 around two barrels before circling the track. The first wagon to cross the finish line usually wins, but time penalties are given for infractions like knocking over a barrel, wagon interference or an outrider finishing too far behind his wagon driver.

The rules also require that the horses be given a mandatory day of rest after every four days of racing.

To reduce crowding on the track and the chance of collisions, the number of outriders alongside each competing chuckwagon was decreased to two from four. Several of the riders were seriously injured in previous years.

At last year's Stampede, safety officials fined chuckwagon driver Cliff Cunningham $12,500 — the steepest fine ever levied on a driver at the event — following a race collision that led to the death of a horse. The initial fine was only $2,500, but the penalty was later raised when investigating officials reviewed footage of the accident and interviewed other drivers involved.

Animal welfare groups have continued to cry foul over the popular Stampede, in spite of recent efforts to appease such organizations. Animal rights activists have long charged that the rodeo amounts to sanctioned animal cruelty.

The Vancouver Humane Society has specifically targeted the chuckwagon races, expressing concern over the safety of the event and noting that more than 50 horses in the races have been killed in the past 26 years.