By Dorothy Tucker

CHICAGO (CBS) — They hid in bathrooms and barricaded themselves in conference rooms for a frightening 12 minutes.

Hundreds of Chicago workers thought a gunman was on the loose in their office building.

Turns out, it was a drill gone bad.

CBS 2 Investigator Dorothy Tucker was on the scene from the beginning, asking what went wrong.

Office workers and police also have a lot of questions. At the top of the list: Why didn’t they know about the drill?

They thought it was a real call, so police acted like it was.  At 1:00  in the afternoon, an army of Chicago police swarming the building at 225 West Randolph looking for the shooter on the 7th floor.

So folks ran. Shaky cell video shows office workers heading to the door. Outside, hundreds ran down the street. Police searched the building, going floor by floor.

Anthony Corra heard the commotion in the hallway but he was inside a conference room on the 4th floor with 40 other people, hiding.

“We stacked four or five table on top of each other to make sure nothing came through,” Corra said. “I’m still kind of shaking.”

Police said they were on high alert for nearly 12 minutes thinking the worst when they finally got the call that it was a drill.

The building management had sent a text on  through the building’s alarm system, but they didn’t notify the right people.

“Building management didn’t clarify to CPD or building that this was taking place,” said CPD 1st District Commander Michael Pigott.

Police still trying to figure out who sent the text, why didn’t notify police or office  managers. The city of Chicago’s law department will review the incident to see if anyone faces fines.

In a statement, and AT&T spokesman said:

“A training class this afternoon caused a false alarm in the building. This was an instructional training video – not a drill or formal exercise.  We’re grateful to the Chicago Police, Fire and Emergency Management teams for their swift response and we are investigating the incident.”

Dorothy Tucker