The US Justice Department will investigate patterns of racial disparity in the use of force by Chicago police officers, the US attorney general has announced.
The investigation comes as prosecutors declined to charge another officer over the shooting of a 25-year-old black man who authorities said was armed with a gun as he ran away from officers.
It follows the release of a video two weeks ago showing a white Chicago police officer shooting another black teenager 16 times.
Police killings of African-American men over the past year have shaken several US cities and given rise to the "Black Lives Matter" protest movement.
The Chicago investigation, which is separate from an existing federal investigation into last year's shooting of 17-year-old Laquan McDonald, will review how the department disciplines officers and handles misconduct accusations.
Justice Department officials say they use such patterns-and-practices investigations to identify systemic failings in troubled police departments and to improve trust between police and the communities they serve.
"This mistrust from members of the community makes it more difficult to gain help with investigations, to encourage victims and witnesses of crimes to speak up, and to fulfil the most basic responsibilities of public safety officials," said Attorney General Loretta Lynch.
"And when suspicion and hostility is allowed to fester, it can erupt into unrest."
Chicago Mayor Rahm Emanuel and the police department are under intense scrutiny over their handling of McDonald's 2014 death.
Officer Jason Van Dyke was charged with first-degree murder last month.
Mr Emanuel initially said a federal civil rights probe would be "misguided" but later reversed course.
In a news release after Ms Lynch's announcement, the mayor said his goal is to create a stronger and better police force "that keeps the community safe while respecting the civil rights of every Chicagoan".
The investigation came as Cook County State's Attorney Anita Alvarez said there would be no charges against Officer George Hernandez in the shooting of 25-year-old Ronald Johnson on October 12, 2014.
Ms Alvarez and Assistant State's Attorney Lynn McCarthy spent more than 30 minutes detailing evidence before showing the video, which similarly to the McDonald case has no dashcam audio; the state's attorney's office overlaid police radio communications.
The video showed Johnson running from police across a street with several officers in pursuit, and then one officer shooting. 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.
She also said Johnson "ignored" officers' commands to stop and drop his weapon and had been in a physical altercation with at least one other officer before he was shot.
Ms Alvarez has been criticised for not filing charges earlier in the McDonald case, in which the video shows the teen veering away from officers on a four-lane 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 and is barely moving.
The video does not include sound, which authorities have not explained.
The Chicago City Council signed off on a 5 million dollar 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.
Since the video was made public, Mr Emanuel forced Police Superintendent Garry McCarthy to resign and formed a task force to examine the police department.
But the calls for the mayor to resign - something he said he will not do - have grown louder from protesters.