IN JANUARY, an investigation by the Justice Department found that the Chicago Police Department routinely used excessive force against the city's residents, often along racial lines and without accountability. That report recommended federal court oversight of the Chicago police to prevent further abuses. Now, almost nine months later, a federal judge is set to begin supervising the process of reforming Chicago's police.

But the city of Chicago won't be working with the Justice Department. Instead, it's Illinois Attorney General Lisa Madigan who is bringing the lawsuit to begin negotiations on a federal court decree for police oversight.

The state of Illinois is filling the hole left by Attorney General Jeff Sessions, under whose leadership the Justice Department pulled back from its agreement to negotiate with Chicago to find a mutually agreeable model for court supervision of the city's police. After months, nothing came of Chicago Mayor Rahm Emanuel's efforts to find a solution with the Justice Department outside the courts. Now, Mr. Emanuel — who has been reluctant to embrace judicial oversight of Chicago police — has pledged to partner with Ms. Madigan to achieve reform under the watchful eye of a judge.

Chicago is one of several cities left behind by Mr. Sessions's emphasis on fighting crime over working with unsettled police departments in need of reform — as if protecting civil rights and public safety were somehow incompatible. Two months into his time as attorney general, Mr. Sessions issued a memorandum directing his deputies to review oversight agreements reached by the Obama administration with police departments found to have systematically violated civil rights. The Justice Department then tried to delay an agreement finalized by Obama officials from going into effect in Baltimore, over the objections of the police department itself — only to be rebuked by the judge who gave the reform plan his approval. And recently, the department's Community Oriented Policing Services Office (COPS) has reportedly failed to provide assessments requested by at least seven local police departments that reached out for help with reform.

The ChicagoPolice Department is less eager for reform than those of Baltimore and the jurisdictions working with COPS. But the heavy reliance of Illinois's court filing on the Justice Department's investigation emphasizes that Ms. Madigan's office is playing the part of a federal government that has fallen silent.

Illinois's lawsuit should be encouraging not only to the residents of Chicago — whose police department may finally receive the oversight it so sorely needs — but to other states also seeking to improve police departments in the absence of federal assistance. The road ahead is untested, and state attorneys general without the relatively broad authorities of Illinois's highest law enforcement officer may find themselves relying on novel legal arguments to bring lawsuits pushing reform. It is heartening to see the Illinois attorney general step into the shoes of the Justice Department. But those shoes never should have been left empty in the first place.