Madigan announced the lawsuit at a news conference Tuesday afternoon alongside Chicago Mayor Rahm Emanuel.
Emanuel was in the process of working with the Justice Department on police reforms at the end of the Obama administration, but has been in a stalemate thus far with Attorney General Jeff Sessions on how to move forward.
"The reforms we have made in recent years, and those that lie ahead, will help us ensure Chicago has the most professional, proactive police department possible," said Emanuel. "I am proud that Illinois' attorney general is standing up -- for our city and our officers -- where the Trump Justice Department fell flat."
The lawsuit specifically cites two damning reports on the Chicago Police Department, saying "that CPD has continued to engage in a repeated pattern of using excessive force and racially discriminatory policing practices."
One report, from the Justice Department during the Obama administration, described the pattern
resulting from "systemic deficiencies in training and accountability, including the failure to train officers in de-escalation and the failure to conduct meaningful investigations of uses of force."
Sessions has expressed great skepticism of federally-mandated police reform measures since taking office in February, especially the use of consent decrees, and to date has not moved on the city's efforts to implement one in Chicago.
The Justice Department said in a statement to CNN Tuesday that "local policing is first and foremost a matter of local responsibility."
"If the city and state can put in place policies and practices to ensure constitutional, proactive policing that actually serves to reduce Chicago's rampant violent crime problems, that would be a positive step," said DOJ spokesperson Ian Prior. "However, we have said repeatedly that we will not agree to or support any measure that will endanger the lives of Chicago's residents or law enforcement by eroding the rule of law or by failing to properly address violent crime in Chicago. We will continue to work with the city to meet those goals."
Emanuel faced opposition from Madigan and city leaders earlier this year when his office proposed a less stringent
plan for police reforms but ultimately could not reach an agreement with DOJ.
But Madigan now asks that the court appoint an independent monitor to oversee Chicago's implementation of the new policies and procedures with court oversight.
Former DOJ officials who worked on the federal investigation into CPD said the lawsuit was a step in the right direction.
"My take is that it is heartening that a government actor is stepping into the void left by Sessions' abdication of his enforcement responsibilities to protect the civil rights of Chicago's residents," said Christy Lopez, a visiting professor at Georgetown Law who formerly served as a deputy chief in the Special Litigation Section of the Civil Rights Division at DOJ and helped lead the CPD investigation.
"Where Attorney General Sessions refuses to act, we hope that other state and local governments will follow Illinois' example and step into the breach," agreed Vanita Gupta, former head of DOJ's civil rights division under Obama, who now leads the Leadership Conference on Civil and Human Rights. "Everyone deserves a police department that will keep them safe, consistent with the protections our Constitution demands."