(CNN)No criminal charges will be filed against the Chicago police officer who shot and killed Ronald Johnson in October 2014, Cook County State's Attorney Anita Alvarez said Monday. Johnson was armed with a loaded gun at the time of the shooting, she said.
At a news conference, Assistant U.S. Attorney Lynn McCarthy played dashboard camera video of the shooting for reporters. The video appeared to show Johnson running away from police officers and into a public park. Out of view of the camera, the 25-year-old was shot twice, she said.
For nearly an hour, McCarthy used a PowerPoint presentation to explain in detail where police cars were and where Johnson ran from officers. The officers were in the area responding to numerous 911 calls from residents saying that shots had been fired. Some of those calls were played at the news conference.
McCarthy explained that Johnson had been in a car with three other people that was shot at, had left the scene and then returned. While officers were interviewing one of the men in that car, Johnson tried to run.
An independent police board reviewed the dashboard camera video, McCarthy said, and decided that Officer George Hernandez was not wrong for shooting Johnson.
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It's impossible to tell, based on the video shown at the news conference, whether there was anything in Johnson's hand because the footage is too blurry and grainy. McCarthy played it at normal speed and then in slow motion. What is clear is that a man who McCarthy said was Johnson is seen running around a corner. Two officers appear to be chasing him, and they round the corner, too. Johnson runs across the street toward a park.
The video shows him on the edge of the park, and out of the camera's range, he was shot.
McCarthy and Alvarez said that officers reported that Johnson had a gun.
McCarthy showed a photo of a gun that she said was taken from the scene and had grass lodged in it. That gun, she said, was tied to a shooting that occurred in Chicago in 2013.
Alvarez called Hernandez's actions "reasonable and permissible" because she said Johnson posed a threat to police and was running, armed, into the park.
A mother pushed for video to be released
Johnson's mother, Dorothy Holmes, who earlier said she had seen the video, has said that it proved her son was murdered. She and her attorney have pushed for the video's release.
They continue to insist that Johnson was not armed.
After Alvarez and McCarthy gave their account, in a separate news conference attorney Michael Oppenheimer accused them of presenting an "infomercial" full of information, but none that conclusively demonstrated that Hernandez was justified in shooting Johnson.
Oppenheimer laughed when a reporter asked about Alvarez's contention that Johnson was a threat because he was heading into a public park. It was after midnight, he noted, dismissing the notion that anyone would be in the park.
The attorney continued to drive home his central argument: "You can see no gun," he said. "There is no gun visible in Ronald Johnson's hand, because there was none."
Holmes spoke after her attorney, saying she was disappointed with the Cook County prosecutor's decision not to charge the police officer.
"I hope one day she feels the pain that I feel," the mother said.
Earlier Monday, U.S. Attorney General Loretta Lynch announced that the Justice Department would be conducting a sweeping investigation of the Chicago Police Department's use-of-force practices and whether there are racial and ethnic disparities in how officers use force.
The push to make the Johnson shooting video public had been months in the making.
Holmes filed a federal lawsuit against Chicago police shortly after her son's death, and the defendants filed a motion to block the video's release. Oppenheimer then filed a Freedom of Information Act request, arguing that the footage was public record. That was denied.
He said last week on CNN that the video shows that Johnson was not carrying a weapon, "nor did he ever turn and point anything."
"The Police Department planted that gun because there's no way anything would have stayed in Ronald Johnson's hand after he was shot," the attorney said.
Chicago police have not responded to CNN's request for comment.
"A life was lost here, and that is a tragedy that can't be taken lightly no matter the circumstances. That's why independent investigations are so crucial in these cases," Chicago Mayor Rahm Emanuel said in a statement Monday.
"Now, as our independent police review authority resumes its investigation to determine whether the shooting was consistent with CPD's police, we must also ask ourselves if the existing policies on the use of deadly force are the right ones and if the training we provide to officers to make split-second decisions in life or death situations is sufficient."
Johnson's case in the light of Laquan McDonald's
Johnson's case is receiving renewed attention after a dashboard camera video was made public around Thanksgiving that showed a Chicago police officer shooting 17-year-old Laquan McDonald.
McDonald's killing occurred eight days after Johnson's.
AG: Justice Department investigating if Chicago police broke law
McDonald is seen in the video walking in the middle of a road toward patrol cars. The teen, who is holding what police said was a 4-inch knife, veers away from the cars, his back to them, and is shot 16 times. The officer, Jason Van Dyke, remained on the police force for more than a year, and the city went to court to prevent the video's release. A freelance journalist sued for it, arguing that the footage was public record. A judge sided with the journalist and ordered that the video be made public.
Van Dyke is white, and McDonald was black.
The video instantaneously sparked outrage and protests for days in Chicago. Demonstrators demanded that Mayor Emanuel and Police Chief Garry McCarthy resign.
Van Dyke was arrested and charged with first-degree murder.
Many people who took to the streets questioned why it took 13 months to release the video. Protesters chanted, "16 shots and a coverup!"
The video was held from the public for more than a year because releasing it risked prejudicing a federal and state's attorney investigation, Emanuel explained amid the backlash. The mayor announced that he fired McCarthy and was setting up an independent panel that would review police training.
Last week, Illinois Attorney General Lisa Madigan said she sent a letter to the U.S. attorney general asking the Department of Justice's Civil Rights Division to investigate the Chicago Police Department to see whether its practices violate the Constitution and federal law.
The Justice Department, which has initiated several such investigations, including in Baltimore and elsewhere, is reviewing the letter.
For more than a year, Black Lives Matter activists and others have tried to call attention to the role of race in policing. They point to cases in New York; Ferguson, Missouri; and Baltimore, where they say police have used excessive and deadly force against black men.
CNN's Dana Ford, Bill Kirkos, Sheena Jones and Holly Yan contributed to this report.