Image copyright AP Image caption Attorney General Loretta Lynch said the investigation would focus on the department's use of force

The Chicago Police Department will be the subject of a wide-ranging federal investigation, US Attorney General Loretta Lynch has announced.

She said the investigation would focus on the use of force by officers and the department's accountability procedures.

The announcement follows weeks of protests over the police killing of a black teenager by a white officer.

Laquan McDonald, 17, was shot 16 times in 2014 by the officer, who was charged with murder over a year later.

Chicago Mayor Rahm Emanuel welcomed the inquiry after initially opposing it.

In a separate and unrelated case, Illinois State Attorney Anita Alvarez said that no charges would be filed against another police officer, George Hernandez, who shot and killed Ronald Johnson, a black man, in October of 2014.

Authorities say he pointed a gun at police and refused to surrender. The man's family say that he was running away, that he was unarmed, and that the gun was planted.

Dashboard video released on Monday shows Johnson running away from police, and he runs out of the frame as the officer opens fire.

Image copyright Getty Images Image caption Regular protests have taken place since a video showing McDonald's shooting was released last month

Ms Lynch, speaking in Washington, said a public report would be released after the investigation and would detail any abuses, if they are found.

She said that the investigators - a team of federal lawyers - would be focusing on the department's use of force and whether there were any "racial, ethnic and other disparities". She also said it would focus on the department's accountability procedures for officers involved in the use of force.

A court-enforceable agreement would be sought if abuses are found, she said, and the Justice Department will work with city officials to reform the department if need be.

The inquiry will take a similar form to those recently conducted in Baltimore and Ferguson, Missouri - as well as more than 20 other departments in recent years, and will look for systematic violations of US civil rights laws.

Mr Emanuel and other city officials are under intense scrutiny following the release of dashboard camera footage in late November that shows the moment McDonald was shot by a white police officer.

Media playback is unsupported on your device Media caption Dashcam footage shows the moment 17-year-old Laquan McDonald was fatally shot by police in Chicago

The officer, Jason Van Dyke, was charged with first-degree murder just a few hours before the video was released - more than a year after the shooting.

The video shows the teenager walking around and possibly away from police before gunshots rang out. The shooting continues even after the teenager is on the ground motionless. Police say that he refused to drop a knife.

Protests have taken place regularly throughout the region since the video's release, with many demonstrators alleging a cover-up and calling for the mayor to resign.

Image copyright Reuters

In response to the outcry, Mayor Emanuel fired the city's police chief and later announced the resignation of the Independent Police Review Authority's chief.

Mr Emanuel originally said a federal investigation would be "misguided", but later back-tracked and welcomed an investigation.

The Chicago City Council settled with the McDonald family for $5m (£3.3m), and officials in the city fought in court for months for the video to be kept under wraps.