An Arizona police officer was fired Tuesday, a day after he shot and killed a 61-year-old man in a wheelchair while working off-duty as a security guard at a Walmart

Tucson Police Chief Chris Magnus told reporters he was "deeply disturbed and troubled" by the actions of Ryan Remington, a four-year veteran of the police force. Authorities released footage of the shooting Tuesday. 

"His use of deadly force in this incident is a clear violation of department policy and directly contradicts multiple aspects of our use of force and training," he said.

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Surveillance video shows Tucson police officer Ryan Remington following a man he shot and killed Monday night.  (Tucson Police Department )

Remington was working a security detail at Walmart on Monday night when an employee alerted him that a customer in a motorized wheelchair had stolen a toolbox.

Remington and the employee followed the customer,  identified as Richard Lee Richards, in the store parking lot.

The employee said Richards was asked to provide a receipt for the toolbox. Richards allegedly pulled out a knife and said" "Here's your receipt," Magnus said, and he continued moving toward a Lowe's home improvement store. 

Remington ordered Richards to put the knife down, which he ignored, Magnus said. As he approached the entrance to the garden section of the Lowe's, Richards was warned by other officers who responded to the scene not to enter. 

Police body-camera footage shows the moments before Richard Lee Richards was fatally shot by a Tucson police officer.  (Tucson Police Department )

When he ignored their commands, Remington fired nine rounds, striking Richards in the back and side and causing him to fall to the ground, Magnus said. 

He died at the scene. Police body-camera footage shows the Richards from behind attempting to enter the Lowe's. A male voice is heard ordering him not to enter before a barrage of gunfire is heard. Richards is seen falling to the ground. 

The case is under review by the Pima County District Attorney's office. 

In a statement to Fox News, Tucson Mayor Regina Romero called the shooting "unconscionable and indefensible."

Lowe's security footage shows Richards in his wheelchair before he was shot.  (Tucson Police Department )

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"The County Attorney’s Office has my full support as they proceed with their investigation," she said. "It is moments like this that test our resolve to ensure justice and accountability. We owe this to all Tucsonans. I ask our community to remain calm and be patient as investigations ensue."

Fox News has reached out to the Tucson Police Officers Association for comment on Remington's firing.