The investigation comes amid rising tensions in the city following the release of a graphic police dashcam video of the October 20, 2014 incident that shows 17-year-old Laquan McDonald shot 16 times by a police officer.
Chicago fires top cop after black teen's fatal shooting
"Our investigation is focused on use of force and accountability," Lynch said.
"We'll be looking at how force including deadly force is handled, investigated and how officers are held accountable for that. That is our focus right now."
A short time later, Cook County State Attorney Anita Alvarez announced in Chicago that based "on the totality of the evidence" she would not bring criminal charges in a separate police-involved shooting of a black suspect, Ronald Johnson, on October 12, 2014.
But with the city's handling of such cases under intense scrutiny, Alvarez defended the decision at a press conference in which she released a police dashcam video and a video presentation with recordings of 911 calls and police radio traffic at the time of incident.
Chicago charges officer in black teen’s death, releases video of shooting
"We are in the digital age and we didn't have videos like this 15 or 20 years ago and now we do," Alvarez said. "And so we as prosecutors are changing the way that we have to do things, because of the fact that we have communities and we have the public that want to know."
Johnson was shot to death while running from police, allegedly ignoring police orders to freeze and drop the 9 mm handgun he was carrying.
Police tactics and racism have been the subject of an intense national debate since protests erupted in Ferguson, Missouri in summer 2014 over the shooting death of another black teen, 18-year-old Michael Brown.
McDonald's death occurred just months later, on October 20, 2014, but did not come to national attention for more than a year, and then only after a court ordered the release of the dashcam video that captured the shooting.
University of Chicago cancels classes over gun violence threat
McDonald, who was holding a knife when he was shot, is seen gunned down in the middle of the street by police officer Jason Van Dyke who continues shooting after the 17-year-old falls to the ground.
Prosecutors said Van Dyke opened fire just 30 seconds after his cruiser pulled up to the scene and six seconds after stepping out of it.
Van Dyke was charged with first degree murder over McDonald's death the same day the video was released.
It was the first time in 30 years that a Chicago police officer was charged with first degree murder for an on-duty fatality.
Chicago Mayor Rahm Emanuel fired police chief Garry McCarthy last week, saying public trust in his leadership had been "shaken and eroded."
Summer of bloodshed as US murder rates rise
In a statement Monday, Emanuel pledged the city's "complete cooperation" with the federal probe.
"Our mutual goal is to create a stronger, better police department that keeps the community safe while respecting the civil rights of every Chicagoan," he said.
Lynch said lawyers from the Justice Department's civil rights division will lead the federal probe into the Chicago police force, which will be wider than a parallel US investigation into the fatal shooting of McDonald.
She said they will look into whether police in the midwestern US city "engaged in a pattern or practice of violations of the Constitution or federal law.
"Specifically, we will examine a number of issues related to the CPD's use of force, including its use of deadly force; racial, ethnic and other disparities in its use of force; and its accountability mechanisms, such as its disciplinary actions and its handling of allegations of misconduct," she said.
If unconstitutional practices are found, the Justice Department will seek court-enforced reforms of the police force, she said.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ