CHICAGO — With federal agents preparing to scrutinize Chicago’s police practices in a wide-ranging civil rights investigation, Mayor Rahm Emanuel is defending measures the city has already taken in the wake of protests over the fatal shooting of a black teenager by a white officer.

Emanuel said the city would press ahead with initiatives of its own just hours after U.S. Attorney General Loretta Lynch announced the Department of Justice investigation into racial disparities and the use of force by Chicago police, saying at a news conference that he was “not going to wait.”

But such assurances are likely to be met with skepticism in a city that’s been plagued for years by a string of incidents of apparent police abuse followed by promises of reform. Several activist groups said they planned further protests in light of a decision announced Monday by Cook County’s top prosecutor not to criminally charge another officer for shooting a 25-year-old black man under different circumstances.

Emanuel said at a Monday afternoon news conference outside his office at City Hall that the shooting of 17-year-old Laquan McDonald by Officer Jason Van Dyke had brought the city to what he called “an inflection point.” A squad-car video of the shooting sparked days of protests, criticism of how Emanuel and other officials handled the incident, and calls for federal intervention.

“It is clear to me when you look at Laquan McDonald and other experiences — and they go way back in history — …, then none of the things we’ve done in the past have measured up to the scope, the scale and consequences of what needs to be done,” Emanuel said.