Chicago prosecutors released dash-cam video on Monday showing the fatal shooting of 25-year-old Ronald Johnson III by a Chicago police officer in October 2014. Police claimed that Johnson had pointed a gun at Officer George Hernandez before he was shot, but the video shows Johnson running away from officers at the time of the shooting.

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Hernandez fired five shots at Johnson about two seconds after exiting a patrol car, killing him. At a press conference on Monday, Cook County State's Attorney Anita Alvarez said that Hernandez would not face criminal charges because Johnson allegedly had a gun, resisted arrest, and posed a danger to the public.

"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. "Even though Mr. Johnson was running away from Hernandez and other police officers, he was running towards a police vehicle containing two other responding officers and unknown numbers of the public inside that park."

Alvarez detailed the police account of the shooting in the 75-minute press conference. Prior to the shooting, Johnson was reportedly leaving a party with three friends when the back window of their car was shot out; they drove around and returned to the scene where police had convened in response to reports of gunfire.

Investigators say that Johnson was seen carrying a gun and broke free when an officer attempted to arrest him. Dash-cam video shows him running away from police when Hernandez and another office pull up in a patrol car. Hernandez fires his weapon five times, and two of the bullets struck Johnson in the back.

Michael Oppenheimer, an attorney for Johnson's family, argues that the video contradicts official accounts of the fatal shooting.

"He never turned, he never pointed, and he was not carrying a gun," Oppenheimer stated. "There was nothing in his hand, not a gun, a cell phone, a bottle of water. Nothing."

But Alvarez maintained that a gun recovered from the scene had Johnson's DNA on it.

This is the second time that officials have released dash-cam video showing a fatal shooting by a Chicago police officer in less than two weeks, the Associated Press reports. Last month, footage of the fatal shooting of 17-year-old Laquan McDonald by Officer Jason Van Dyke prompted protests throughout the city. The U.S. Justice Department announced Monday that it was launching an investigation into the Chicago Police Department in order to determine whether the department had violated use of force and deadly force policy.

In a statement, Chicago Mayor Rahm Emanuel vowed to cooperate with the federal investigation.

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"Our mutual goal is to create a stronger, better Police Department that keeps the community safe while respecting the civil rights of every Chicagoan," Emanuel said Monday. "Nothing is more important to me than the safety and well-being of our residents and ensuring that the men and women of our Police Department have the tools, resources and training they need to be effective crime fighters, stay safe, and build community trust."