Associated Press

CHICAGO — Responding to deepening mistrust of one of the nation's largest police forces, the U.S. government opened an investigation Monday into the Chicago Police Department, while authorities said they would not charge an officer in the death of a 25-year-old black man who was shot in the back last year.

The Justice Department investigation will look into patterns of racial disparity in the use of force. It comes nearly two weeks after the release of a video showing a white Chicago police officer shooting a black teenager 16 times.

Lack of trust between police and their communities "makes it more difficult to gain help within investigations, to encourage the victims and the witnesses of crime to speak up and to fulfill the most basic responsibilities of public safety officials," Attorney General Loretta Lynch, the U.S.'s top lawyer, said. "And when suspicion and hostility is allowed to fester, it can erupt into unrest."

The investigation, which is separate from an existing federal investigation into last year's shooting death of 17-year-old Laquan McDonald, will also review how the department disciplines officers and handles misconduct accusations.

The civil-rights investigation follows recent ones in Baltimore and Ferguson, Missouri, and comes as the police department and Mayor Rahm Emanuel are under intense scrutiny over their handling of the October 2014 death of McDonald. Officer Jason Van Dyke was charged with first-degree murder Nov. 24, more than a year after the killing and just hours before the release of police dashboard camera footage showing the officer shooting the teenager.

Emanuel, who initially said a federal civil rights investigation would be "misguided" but later reversed course, said the city needs comprehensive solutions.

Speaking at a news conference, Emanuel said he's making several reforms, including appointing a new leader for the Independent Police Review Authority, which investigates shootings by police. The previous head resigned Sunday.

Also Monday, Cook County State's Attorney Anita Alvarez said there would be no charges against Officer George Hernandez in the shooting of Ronald Johnson. Authorities say he pointed a gun at police before he was killed on Oct. 12, 2014.

Alvarez and Assistant State's Attorney Lynn McCarthy spent more than 30 minutes detailing evidence before showing the dashcam video, which shows Johnson running from police across a street with several officers in pursuit, and then one officer firing. Johnson is not on screen when he was struck by two bullets.

The video was also slowed down to show what McCarthy said was a gun in Johnson's hand. Prosecutors say a loaded weapon was found in his hand after he was killed. She also said Johnson ignored officers' commands to stop and drop his weapon.

The attorney for the Johnson family, Michael Oppenheimer, said the prosecutors' investigation was a "joke" and an affront to Johnson's family and Cook County citizens.

When asked why there was no audio on this video and others, Alvarez said: "That's a problem for the Chicago Police Department, and I think they need to answer to that. ... Time and time again we look at these videos, and there is not any audio."

Alvarez has been criticized for not filing charges earlier in the McDonald case, in which the video shows the teen veering away from officers on a street when Van Dyke, seconds after exiting his squad car, opens fire from close range. The officer continues shooting after McDonald crumples to the ground.

The Chicago City Council signed off on a $5 million settlement with McDonald's family even before the family filed a lawsuit, and city officials fought in court for months to keep the video from being released publicly. The city's early efforts to suppress the footage coincided with Emanuel's re-election campaign, when the mayor was seeking African-American votes in a tight race.

Since the release of the McDonald video, Emanuel forced Police Superintendent Garry McCarthy to resign and formed a task force to examine the police department. But protesters' calls for the mayor to resign — something he said he won't do — have grown louder.

Politicians, including Democratic presidential candidate Hillary Clinton, called for the federal civil rights investigation.

The Justice Department has opened 23 investigations of police departments since the start of the Obama administration.

The federal government has the option of suing a police department that is unwilling to make changes.

Copyright 2015 The Associated Press