Chicago Police responded to an "active shooter call" in Willis Tower in the city's Loop on Wednesday, but later found the reports to be unfounded, according to authorities.
The call came from a conference room for a company that operates three floors of the skyscraper, located at 233 S. Wacker Dr., police said.
Employees subsequently heard over an intercom "this is not a drill" and proceeded to use active shooter training skills, officials said. The call came from across an internal phone system of the company, though it was unclear whether it was a prank or mistake.
After employees called police, officials said the Chicago Police Department and SWAT responded to the scene and took the freight elevator to the 50th floor. No threat was found.
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Police cleared the three floors belonging to that company of employees, authorities said.
According to authorities, the company is doing its own audit and investigation of the incident. A non-criminal report was filed by CPD, explaining why and how officers responded to the call.
When asked about the activity at Willis Tower, Chicago Police Supt. David Brown said during a press conference Wednesday that officers found any reports of an active shooter to be unfounded.
"Not accurate reports of any kind of active shooter -- debunked it almost immediately. Taking good care and ensuring that there’s no other activity currently," Brown said.
Brown added that in light of the county's mass shootings, police "are taking every precaution," following all normal protocols until able to debunk a report.
Willis Tower management on Wednesday said in an email to those who work inside that the building "remains safe."
"To all Willis Tower tenants, please be advised that reports of an active shooter in the building are false." There may be a police presence in the building as we conclude this situation, but the Willis Tower remains safe," the email read.