A 3-year-old dog named Chloe escaped from a garage on the 9600 block of Nucla Street.
A Commerce City police officer shot the dog five times as an animal control agent placed a "catch pole" around its neck.
Sean Miller is a professional trainer hired by Commerce City police in 2010 to teach officers how to avoid using deadly force against animals.
He was hired after a similar incident in which a family dog was shot and killed by an officer.
"It's very unfortunate that a dog had to lose its life in this circumstance," Miller said.
Miller said the video of Saturday's incident, taken by a neighbor, shows Chloe was likely not a threat.
"It appeared that the dog was actually in 'flight mode' which means she is actually fleeing the situation," Miller said.
Miller said deadly force is necessary less than two percent of the time.
"Any time there is lethal force used in a situation like this, it is likely excessive force. I can't say with 100 percent certainty that this officer used excessive force," Miller said.
Miller said police did the right thing by using a stun gun on Chloe, which they say had little effect.
Miller said other non-lethal options would have been pepper spray, and that "catch pole" around the dog's neck.
At the Commerce City civic center Monday evening, more than two dozen people gathered for a candlelight vigil and demanded Commerce City police be held accountable for their actions over the weekend.
"Because of some negligen, that dog is no longer with us," one protester said.
As outrage grows over the Chloe's death, the incident brings to mind another shooting two years ago.
A Commerce City officer checking on a 911 hang-up call said three vicious dogs threatened her and she had no choice but to shoot the Agazio family's dog, Zoey.
Attorney Jennifer Edwards with the Animal Law Center sued on behalf of the Agazio family, and lost.
In August, a jury ruled the use of deadly force was justified.
"It's absolutely infuriating," Edwards said.
Edwards said the 2010 case also found killing someone's pet could be a violation of the owner's constitutional rights.
"You would think knowing that they would have been far more careful," Edwards said.
In a written statement, Commerce City Chief of Police Chuck Saunier addressed Saturday's incident.
"Obviously, this kind of force is a last resort for our officers. It truly is a tragedy any time a member of our department has to use their weapon to ensure community safety. Our initial review of this incident indicates that the officers responded appropriately to the 911 call and used the force necessary to protect the public," Saunier said. "I have ordered a full investigation of this incident that is already underway. I am optimistic it will determine all of the facts of this tragic event. The home video is disturbing, however until the investigation is complete, it is inappropriate to speculate on the incident because we don't have all of the facts."
Commerce City police say they tried for 20 minutes to reach the family who was watching the dog Saturday before attempting to take it to the shelter.
That dog sitter was cited for a pit bull violation and for having a vicious animal.
A police spokesman said, if their investigation uncovers any wrongdoing on the officers' part, they'll own up to it.
"The idea of dogs is changing. They are no longer just pets, they are family members," Miller said.
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