The Chicago Police department shows a pattern of using excessive force and civil rights violations, according to the year-long report released by the US Department of Justice.

The investigation found that the police department violated the constitutional rights of residents for years, permitting racial bias against blacks and killing people who did not pose a threat.

The use of unreasonable force "falls heaviest on predominantly black and Latino neighbourhoods," the justice department said in a statement accompanying the release of a 164-page report.

Also, officers often maintained a "code of silence" to thwart investigations into misconduct.

The investigation was spurred by a dashcam video showing an officer shooting Laquan McDonald 16 times as he was running away. A video that authorities waited more than a year to make public, sparking a wave of anger.

"The failures we identified in our findings have deeply eroded community trust," said Principal Deputy Assistant Attorney General Vanita Gupta, head of the Civil Rights Division.

Chicago police officers shot people who did not pose an immediate threat and used stun guns on people for no other reason than they did not obey the officers' verbal commands, Gupta said in a press conference on Friday.

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Attorney General Loretta Lynch said that the Chicago police lacked proper training and that the department does not adequately look into officers' use of force. All those problems were "compounded by poor supervision and oversight, leading to low officer morale."

Following the report, the justice department and the city of Chicago have signed a joint statement in which the nation's third largest city commits to police reforms under a court-enforced agreement, called a consent decree. The series of reforms are to be supervised by an independent monitor.

"This isn't a new story – both in the short or long view. The report confirms what other reports have already said in the past couple of years ... Those have not magically caused a radical transformation of the CPD," said Simon Balto, a history professor and author on policing practises.

The agreement in principle provides only broad outlines for reform, including commitments to improved transparency, training and accountability for bad officers. Officials from the justice department and city will negotiate a final settlement.

Those talks will take place under the incoming administration of President-elect Donald Trump.

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Trump has vocally supported aggressive law enforcement. And his nominee to succeed Lynch at the justice department, Jeff Sessions, has criticised the sort of consent agreement Chicago is now negotiating with federal officials.

"One of the things that's most concerning to me about Sessions is that he may use DOJ not for reports like the one on the CPD, but against black protest groups. In 2015, he expressed concern that DOJ wasn't investigating what he took to be anti-police hate speech by Black Lives Matter activists," Balton said.

Under the new administration, it falls on the city's civil-rights activists, organisations and community leaders to collaborate in making sure change is enacted, said Timna Axel from the Chicago Lawyers' Committee for Civil Rights.

Chicago Mayor Rahm Emanuel stated his decision to increase the number of lieutenants and other supervisors. He said use-of-force policies were being upgraded, and every officer would be equipped with body cameras and with Tasers to be used "as a less-lethal force option".

The justice department, in the course of its investigation, interviewed and met city leaders, police officials and officers, and residents. It studied investigative files on use-of-force incidents that included more than 170 officer-involved shootings.

Chicago has faced a surging problem with violence in recent years. There were 750 homicides there last year alone, a 10-year high, and more than 3,500 shootings.