A Chicago police officer who shot and killed a black man who was running away from police will not be charged, Cook County State's Attorney Anita Alvarez said on Monday.

The decision comes on the same day that the Justice Department announced that it was launching an investigation into the Chicago Police Department's use of force and deadly force.

During the press conference, Alvarez played a dash-cam video that showed Officer George Hernandez stepping out of his unmarked vehicle to join the pursuit for Ronald Johnson.

Hernandez fires five times quickly thereafter hitting Johnson twice, once in the back of the knee and another in the back of the shoulder.

"The fact that the officer shot Ronald Johnson in the back is one of many factors that had to be carefully considered and evaluated," Alvarez said.

Alvarez explained that Hernandez could have felt threatened even if Johnson was running away. She showed video of another case, which was cited by Hernandez in statements to investigators, where a suspect running from police shot an officer without turning around.

Also, Alvarez explained, the law allows the use of lethal force if an officer believes the suspect poses a threat to others. Alvarez said Johnson was running toward another police vehicle inside a public park.

"I have a tough job," Alvarez said. "I think I have the hardest job in this county. I have to make decisions like this every day."

The shooting happened in 2014. Johnson's family attorney Michael Oppenheimer has said that Johnson did not have a gun at the time of his death.

Police, he told The Chicago Tribune, planted the gun.

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