CHICAGO (Reuters) - The U.S. Justice Department said on Monday it will investigate Chicago’s police department following protests over the 2014 police shooting death of a black teenager, on the same day local prosecutors said they would not seek charges in another police shooting case.

U.S. authorities will look at the department’s use of force, including deadly force, among other issues, U.S. Attorney General Loretta Lynch said at a news briefing.

“Our goal in this investigation ... is not to focus on individuals but to improve systems,” the United States’ top law enforcement official said.

She said federal officials would be investigating “constitutional violations” in one of the nation’s largest police departments.

Lynch’s announcement came after almost two weeks of protests in Chicago following the release of a 2014 police squad car dashboard video showing police officer Jason Van Dyke emptying his gun into 17-year-old Laquan McDonald, shooting him 16 times. Van Dyke, who is white, was charged late last month with first-degree murder.

High-profile killings of black men at the hands of mainly white police officers in U.S. cities have prompted a national debate and protests about the use of excessive force by police.

Two hours after Lynch’s briefing, Cook County State’s Attorney Anita Alvarez gave a detailed presentation to reporters explaining why she would not seek charges in another 2014 police shooting death of a black man.

But Michael Oppenheimer, an attorney for Johnson’s family, called Alvarez’ explanation a “joke,” and protests over the shooting were expected in the city later Monday evening.

Alvarez, who has been under fire for her handling of the McDonald case, said that Ronald Johnson III, 25, was holding a gun and fleeing arrest when he was shot by Officer George Hernandez on Oct. 12, 2014.

U.S. Attorney General Loretta Lynch makes remarks at a news conference to announce a law enforcement action relating to FIFA, as U.S. Attorney for the Eastern District of New York Robert Capers listens, in Washington, December 3, 2015. REUTERS/Mike Theiler

Prosecutors on Monday showed police car dashboard video to reporters and played audiotapes of police radio communications and 911 emergency calls.

The grainy video, which has no audio, showed Hernandez firing at Johnson as he runs into a park. Alvarez said that Johnson had been asked repeatedly by multiple officers to drop his weapon, and that a 9mm semiautomatic pistol was found with Johnson after he was shot.

Prosecutors showed reporters an image with a red-colored circle around Johnson’s hand, saying that forensic experts clarified images of a weapon.

Oppenheimer said that Johnson did not have a gun and mocked Alvarez for relying on an Independent Police Review Authority investigation of the case. Chicago Mayor Rahm Emanuel ousted the head of IPRA over the weekend.

Alvarez said officers were at the scene due to an earlier shooting. Another officer had tried to arrest Johnson, but he had pulled away, knocking the officer over, before running off, Alvarez said.

Alvarez said that based on a review of the evidence and the law, “the prosecution could not establish beyond a reasonable doubt that the actions of Officer Hernandez were not reasonable and permissible.”

Responding to questions about why Johnson was shot in the back, Alvarez said Johnson could have turned and fired at Hernandez or other officers, and that his gun was linked to a 2013 shooting.

Emanuel, who is under pressure from critics over the McDonald case and initially disagreed with calls for a federal civil rights investigation, said on Monday that Lynch would have the city’s “complete cooperation.”

Emanuel ousted his handpicked police superintendent last week.

Emanuel said on Monday that the Johnson shooting was a “tragedy” and that IPRA, under new chief administrator Sharon Fairley, would resume its investigation to determine whether the shooting was consistent with Chicago police policy.

The 32 members of the Illinois legislature’s black caucus on Monday said the federal investigation of Chicago police should also include IPRA and Alvarez’s office.

The Justice Department investigation of Chicago police follows other high-profile investigations of departments in Ferguson, Missouri and in Cleveland. Baltimore police are also under federal scrutiny.