Calgary police say they will not lay charges in last week's tragic accident that saw nine rodeo horses fall to their deaths as they crossed a bridge.

Animal rights groups wanted prosecutors to lay charges against a Calgary Stampede official. But police say the Criminal Code requires cruelty to animals to be "wilful" and they say the accident did not meet that test.

The Calgary Humane Society is still investigating the incident and has said it may lay charges under the Alberta Animal Protection Act.

The horses fell from a bridge over Calgary's Bow River at the end of a 200-kilometre trail ride last week that was organized to celebrate Alberta's 100th anniversary.

As they were corralled through a dark underpass, some of the 200 horses got spooked by a passing train, and the animals rushed onto the bridge. In the stampede, nine of the animals panicked and lept into the river about 10 metres below.

Three died on impact. The others drowned after they were swept downstream. One was put down later because of its injuries.

The horses were to take part in the annual Stampede, which began today. They were to be used as bucking stock in the riding events of the Stampede's world-famous rodeo.

Stampede officials are conducting their own investigation. Spokeswoman Brenda Hanchar said people involved in the ride, as well as witnesses to what happened, are being interviewed.