CHICAGO: As the shrill sound of whistles echoed through a parking garage in Chicago’s North Side on Tuesday, two people flung their car doors open, ducked inside and shrank down into their seats. Outside, a convoy of federal immigration enforcement vehicles that had arrived in the area just minutes prior sped off.

“We just saw a bunch of guys with whistles that chased them out,” said Luke, a landscaper who was working nearby and declined to share his full name. The Trump administration in early September launched a targeting what it said were hardened criminals among immigrants in the US without legal status, though many noncriminals have been swept up in raids.

Since then, the piercing blow of a whistle has become a Chicago-wide means of signaling that US Immigration and Customs Enforcement or US Customs and Border Protection agents are present. It warns undocumented people to flee and invites US citizens to come to the scene to record arrests, give detainees legal information and discourage agents from lingering. The aggressive immigration enforcement effort — which has no end date — has sparked widespread protests and resentment among residents. Hundreds of federal agents have fanned across the third-largest city in the US and its suburbs, often carrying assault rifles and wearing military fatigues. Agents have teargassed crowds, rappeled from a Black Hawk helicopter to raid an apartment building, dragged immigrants from cars, held people at gunpoint and shot two people, including one fatally.