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.