The Cook County State’s Attorney, Anita Alvarez, released dashboard-camera video of the police shooting of Ronald Johnson III last year and announced that she would not charge the officer.  CHICAGO — For days, officials here have faced scathing criticism for taking too long to share what they knew about the death of Laquan McDonald, who was shot 16 times by a police officer in October 2014 as a dashboard video camera captured it all. On Monday, the authorities seemed determined to make a sharply different impression as they released video of another fatal police shooting of a black man from the same month, which has also become a source of contention in the city.  In an hourlong presentation that included audio from 911 calls, detailed maps, witness interviews and dashboard video, the Cook County state’s attorney, Anita Alvarez, said that her office would not bring charges against Officer George Hernandez in the fatal 2014 shooting of Ronald Johnson. Ms. Alvarez said the array of material provided clear evidence that Mr. Johnson had been armed with a gun, resisted arrest and posed a potential threat to pursuing officers.  But the elaborate nature of the presentation also underscored the intense public pressure and distrust local officials, from the Police Department to City Hall, have faced since the release last month of the video of Mr. McDonald. That dashboard video outraged many Chicago residents by showing that Mr. McDonald was veering away from an officer when he was shot, and that the officer continued firing bullets into him after he had already fallen to the ground.  Scrutiny of the Chicago Police Department is also coming from the United States Justice Department, which announced on Monday that it had opened a sweeping investigation into possible civil rights abuses, including whether there have been racial disparities in the department’s use of deadly force.  For months, city officials had fought to prevent the McDonald video from becoming public, saying they did not want to hinder investigations. Until late last week, they had similarly worked to block the release of the video in Mr. Johnson’s case.  “I’m not hiding anything,” Ms. Alvarez said of the Johnson case on Monday.  Several dozen protesters gathered Monday night on a South Side street near where Mr. Johnson was shot. The group marched from the site, chanting “Justice for Ronnieman” and “No justice, no peace.” A local Black Lives Matter organizer deemed the findings in Mr. Johnson’s death a cover-up, called for a special prosecutor to re-examine the shooting and demanded the resignation of Ms. Alvarez and Mayor Rahm Emanuel.  The Cook County state’s attorney, Anita Alvarez, announced on Monday that no charges would be filed against the police officer who fatally shot Ronald Johnson in 2014.  A lawyer for Mr. Johnson’s family said that Officer Hernandez should have been charged with murder, accusing the Chicago police of planting the gun and of falsifying reports. The lawyer, Michael Oppenheimer, described Ms. Alvarez’s lengthy news conference on the case as an “infomercial.” And he called the authorities’ handling of the case “a cover-up from the beginning.”  “He had no right to shoot him in the back, no right at all,” Mr. Oppenheimer said. “He wasn’t carrying a weapon.”  The video shows officers with guns drawn chasing Mr. Johnson, who was 25. At one point, he sprints behind an unmarked police car and runs toward a park. Three police officers then jump out of the unmarked car with guns drawn, and flashes can be seen coming from Officer Hernandez’s weapon as Mr. Johnson is running away.  Ms. Alvarez said enhanced images from the video suggest Mr. Johnson was holding a gun in his hand, and the police reported immediately after the shooting that they had found a Browning 9-millimeter pistol with a 21-round magazine in his right hand. She said that someone Mr. Johnson had been riding in a car with just before the shooting occurred heard him cock his gun.  Mayor Rahm Emanuel of Chicago appeared with the interim police superintendent to defend the actions taken by the city after the shooting death of Laquan McDonald.  Prosecutors say Officer Hernandez fired five shots. One hit Mr. Johnson in the back of the knee, another hit him in the back of his shoulder and exited near his eye.  “Based upon an objective review of the evidence and the law, we have determined that the prosecution could not establish beyond a reasonable doubt that the actions of Officer Hernandez were not reasonable and permissible under the laws of the State of Illinois,” Ms. Alvarez said.  Ms. Alvarez, who has faced intense criticism for waiting 13 months to bring a murder charge against Jason Van Dyke, the police officer in the case of Mr. McDonald, is facing a difficult re-election fight in March.  Mr. Johnson’s mother, Dorothy Holmes, who has sued the city, maintained that her son posed no threat to the police and criticized the prosecutor for taking more than a year to announce a decision in the case. Mr. Johnson died on Oct. 12, 2014, just eight days before Mr. McDonald was shot to death by the police in a different neighborhood, on the Southwest Side.  “I’m not going to stop until I get what I want for him, and that’s justice,” Ms. Holmes said on Monday. “If that had been anybody in her family that got killed like that,” she added, referring to Ms. Alvarez, “that officer would have been charged with murder, and that case would have been looked into way before a year.”  The new Justice Department investigation into the patterns and practices of the Chicago Police Department was announced in Washington by Attorney General Loretta E. Lynch. Though largely the result of fallout over the McDonald case, the federal investigation will look beyond his shooting to examine systemic patterns by the Chicago police, including racial stereotyping and other possible abuses.  A similar investigation by the Justice Department into the police and municipal court in Ferguson, Mo., after the police shooting death last year of an unarmed black teenager, led to a searing report showing numerous abuses.  The Obama administration, buffeted by nationwide unrest over police conduct, has opened about two dozen investigations into law enforcement agencies, including in Baltimore and Los Angeles. But the Chicago investigation is one of the biggest ever opened by the Justice Department into reports of police abuse.  The Justice Department investigation comes in addition to a separate federal investigation into the McDonald case that is expected to review allegations of missing surveillance video and the statements of other police officers who had corroborated Officer Van Dyke’s version of the shooting. Mr. McDonald was carrying a three-inch knife, and Officer Van Dyke said he feared for his safety.  “Building trust between law enforcement officers and the communities they serve is one of my highest priorities as attorney general,” Ms. Lynch said.  At City Hall, Mr. Emanuel, who at one point last week suggested that a Justice Department review of the city’s police force would be “misguided,” said on Monday that he now welcomed the review. “It’s in our self-interest as a city,” he said. “We are at an inflection point.”  Mr. Emanuel spoke in a news conference beside the newly appointed head of the Independent Police Review Authority, which investigates police shootings, and the city’s interim police superintendent. Both of their predecessors have been ousted in the days since the McDonald video was released.  “I’m taking responsibility for what happened,” Mr. Emanuel said, “and I’m taking responsibility for fixing it.”  But as reporters pressed Mr. Emanuel and the interim police superintendent, John J. Escalante, on their handling of the McDonald case, the mayor cut off the news conference and led his fellow speakers out the door while journalists continued to shout questions.  Eric Lichtblau contributed reporting from Washington.  A version of this article appears in print on December 8, 2015, on page A1 of the New York edition with the headline: Another Video From Chicago, but No Charges. Order Reprints| Today's Paper|Subscribe
The announcement followed several weeks of uproar in Chicago over charges for another officer in a separate shooting death, of Laquan McDonald.