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DENVER (KDVR)— Protesters took to the streets in cities across the United States on Thursday in response to a disturbing video showing a Minneapolis police officer pushing his knee into the neck of a black man.

That man, George Floyd, later died as a result of the officer’s actions, according to officials.

The mayor of Minneapolis called the killing a murder. Four officers, involved in the incident, have been fired.

Denver’s police chief, Paul Pazen, said he was in disbelief when he saw the viral video.

“What I saw on the video was clearly wrong,” Pazen said.

The video shows the final moments of Floyd’s life while under the knee of a Minneapolis officer. The officer’s actions are being widely described as horrible, disgusting and outrageous.

In a tweet, Denver’s mayor, Michael Hancock, called for murder charges for all of the officers involved.

Pazen said Denver’s use-of-force policy clearly prohibits the knee-to-neck maneuver.

“The Denver community has worked hard to try to prevent these types of situations from happening in our city,” Pazen said. “This city cares about progressive use-of-force policies.”

But regardless of current DPD policy, Pazen said his department will need to learn from the tragedy in Minnesota.

“The shame would be if we don’t learn from this,” he said.

Police use-of-force experts agree.

“I’m not making any effort to defend the officer,” said Dr. Joel Shults, former chief of police at Adams State University. “I am making an effort to defend the process.”

That process is an evidence-based investigation that police advocates say must be respected and protected. Calls are also growing louder for police agencies to review department policies.

“If this is behavior that we don’t want to continue, and we want to understand this behavior and prevent it in other places, we really need to be taking a system’s perspective and a system’s lens to why these events occur,” said Paul Taylor, a former police officer and current assistant professor of criminal justice at University of Colorado Denver.