(CNN) Chicago Mayor Rahm Emanuel said Tuesday he has asked for the resignation of Chicago Police Superintendent Garry McCarthy.

The announcement at a news conference came amid angry protests in Chicago over the way the city responded when a white police officer shot a black teenager 16 times in October 2014. Dashboard camera footage of Laquan McDonald's killing was released last week after a judge ordered it be made public.

"Superintendent McCarthy knows that a police officer is only as effective as when he has the trust of those he serves," said Emanuel, speaking at City Hall.

McCarthy was not at the news conference. But the mayor's office told CNN the superintendent had, in fact, resigned.

The mayor went on to describe a new task force on law enforcement accountability that will review how the city trains and oversees its police officers. It will include five Chicagoans who have been leaders in the justice system. Chicago native and former Massachusetts Gov. Deval Patrick will be a senior adviser to the group, Emanuel said.

Later Tuesday, Illinois Attorney General Lisa Madigan said she sent a letter to the U.S. attorney general asking the Department of Justice's Civil Rights Division to open an investigation into the Chicago Police Department to see whether its practices violate the Constitution and federal law.

"Trust in the Chicago Police Department is broken," Madigan said in a statement. "Chicago cannot move ahead and rebuild trust between the police and the community without an outside, independent investigation into its police department to improve policing practices."

The turmoil in Chicago isn't unique to the city. For more than a year Black Lives Matter activists and others have tried to call attention to an assertion that some police across the country discriminate against black people. They point to cases in New York; Ferguson, Missouri; Baltimore and other cities, where they say police have used excessive and deadly force against black males.

In Chicago, the outrage has been focused on the killing of McDonald . Dashboard camera footage from October 2014 shows the 17-year-old walking in the middle of a street toward squad cars while holding a knife. He then veers away and turns his back to police, and immediately is shot multiple times.

Jason Van Dyke, the officer who shot McDonald, has been charged with first-degree murder.

Emanuel said Tuesday that he and the superintendent began talking about how the McDonald case had "shaken" public trust in the police. He said that in asking for McCarthy's resignation, his goal was to "confront the challenges" that face the department and "go forward" with "fresh eyes and new leadership."

The mayor was asked by reporters about why it took more than a year for the video to be made public.

It was released after a freelance journalist filed suit, arguing that the video was public record. The city went to court to prevent its being released, arguing that doing so would interfere with a federal investigation and a probe by the state's attorney. A judge sided with the journalist, and the footage was released last week.

Emanuel said Tuesday the video wasn't released earlier because to do so would have interfered with the state and federal investigations.

It's unclear how much the mayor's statements will affect how Chicagoans feel.

Protesters, including prominent African-American leaders such as U.S. Rep. Bobby Rush, D-Illinois, have rallied day after day. Last weekend, they peacefully interrupted Thanksgiving holiday shopping.

"Stop the cover-up!" they shouted, and "16 shots! 16 shots!"

Protesters have repeatedly demanded that Emanuel resign, the chief of police be fired and a federal investigation examine the behavior of public officials involved in the case.

Officer out of jail

Van Dyke was charged with first-degree murder last week when the video was released.

He walked out of jail on bond Monday evening after a judge set his bail at $1.5 million. He is suspended from the Chicago Police Department.

Van Dyke had a history of complaints before he shot McDonald, and in almost every case, he was cleared. The allegations mostly involve excessive force, and at least one complaint alleges that he used a racial slur.

There appear to be no criminal proceedings against Van Dyke before last week, but a jury did award a Chicago man $350,000 after determining that Van Dyke used excessive force during a traffic stop. (The city of Chicago also gave McDonald's mother, who had not yet filed a lawsuit, $5 million in April.)

Meanwhile, an online threat caused fear in the city when a man allegedly said he would shoot white men to avenge McDonald's death. Jabari Dean, 21, was arrested and accused of threatening to kill students and staff at the University of Chicago.

According to a criminal complaint, Dean posted the threat on social media this past weekend.

"This is my only warning. At 10 a.m. on Monday mourning (sic) I am going to the campus quad of the University of Chicago. I will be armed with a (sic) M-4 Carbine and 2 Desert Eagles all fully loaded. I will execute aproximately (sic) 16 white male students and or staff, which is the same number of time (sic) Mcdonald (sic) was killed," the post read.

"I then will die killing any number of white policemen that I can in the process. This is not a joke. I am to do my part to rid the world of the white devils. I expect you to do the same."

Dean is charged with transmitting a threat in interstate commerce. He is facing up to five years in prison.