After nightfall, a few hundred protesters had gathered outside the suburban police department, guarded by hundreds of officers in riot gear.

The measures take aim at a chronic sore point in many communities, particularly among Black residents: that police departments traditionally have little oversight outside their own internal review systems, which often clear officers of wrongdoing in fatal civilian shootings.

The standoff threatens to turn the nationwide protests over the deaths of George Floyd, Breonna Taylor and others into another moment that galvanizes the nation but leaves lawmakers unable to act.