Reports of gunfire outside the Kluczynski Federal Building in downtown Chicago prompted a major police response early on Tuesday, although authorities had not confirmed that a shooting had taken place.
Officers were seen searching for bullet casings near the federal building in the Loop after reports of gunfire at about 4.45am local time, according to local reports.
Chicago Police and Homeland Security vehicles were stationed around the building, while a courtyard was sealed off with yellow and red tape. At least three evidence markers could be seen close to the entrance of what appeared to be a parking garage.
Investigators examined a vehicle and a nearby bag, though it was not immediately clear whether the bag was connected to the inquiry. One person was seen being led away in handcuffs.
Police activity was reported in the 200 block of South Dearborn Avenue, with officers also visible along Clark Street. Traffic continued to move through the area, although police vehicles occupied the first two lanes.
The Chicago Police Department and Homeland Security had not released further details. The investigation remained active and no injuries had been reported.
