Workers panic as Chicago police respond to active shooter drill in the Loop

An unannounced active shooter drill at a Chicago office building caused worker panic and an aggressive response by police.

Officials say police received a call Thursday afternoon of workers hiding because of a shooter on the seventh floor of the 30-story building. Police officers carrying shields entered the building and split into teams on the floor, where they found no evidence of gunfire or that anyone had fired a weapon.

Police say a group of workers in a conference room didn't hide until they saw law enforcement officers. Grim-faced workers were later seen leaving the building.

More than an hour after getting the call, officers were checking each floor for people who locked or barricaded themselves into bathrooms and conference rooms.

Police Cmdr. Michael Pigott called the situation an active shooter drill gone terribly wrong. He says officials will determine how building management "can do better next time."