Police quickly responded, evacuated the building, and shut down several streets.
It turned out a training class caused the commotion.
RAW VIDEO: Concerns of active shooter in Chicago's Loop spark heavy police response
AT& T, which has offices in the building says an instructional video led to a false alarm.
Chicago Police say they were never informed of any sort of training taking place at the building.
Whether classes or drills, botched emergency sessions are a recurring problem that our ABC7 I-Team has investigated for several years. Thursday's incident in Chicago's Loop is just one of many.
When police swarmed the AT& T building it looked and felt real; and CPD officials said for ten to twelve minutes they thought it was an actual active shooter.
"I was scared" said Hayley Karp who works in the building. "People were all scared, trying to figure out what to do."
Most had no clue, including police for a time.
"Proper notification was not made to all people in the building" said CPD first district commander Michael Pigott. "It was a drill it was just not conducted properly."
And the I-Team has found that it is happening with some regularity.
April, 2017: Waubonsee Community College in Sugar Grove. Dozens of police, fire and rescue crews raced to the campus after 9-1-1 calls of an active shooter.
But as the I-Team discovered, there was no shooter in 2017-just an Illinois Terrorism Task Force program funded by FEMA that a few people took too seriously.
A similar scenario played out at Walter Reed Medical Center in Maryland, November 2018.
Military police tweeted an active shooter alert; there was panic. It turned out to be a drill.
A few months earlier at Wright Patterson Air Force Base in Dayton Ohio a training exercise mistakenly put security forces on high alert; storming one building with full weapons.
"An unknown individual calling 9-1-1- believing that there was a real world incident occurring within the base hospital" said Col. Thomas Sherman, commander of the 88th Air Wing.
After Thursday's chaos in Chicago, police officials said they encourage active shooter drills...but "there are safety protocols...we should be notified before a drill is taking place" said CPD commander Pigott. "All the parties in the building should be clearly aware before a drill is taking place so we don't have this."
Late Thursday AT&T released a statement concerning the incident. Officials of the telecom giant-sole tenant in the 31 story building-said there was no drill or formal exercise underway before the Chicago fiasco. Had there been, they said, building employees would have been notified of such a drill.
AT&T officials now claim it was a training class and instructional video that cause the quote false alarm.