OAKBROOK TERRACE, Ill. (CBS) — A misunderstanding on the phone led to a massive police response at a ComEd office building in Oakbrook Terrace on Tuesday.

WBBM Newsradio’s Mike Krauser reports there was a planned fire drill Tuesday morning, and an alarm went off while a customer was on the phone with a ComEd employee.

Hearing the commotion, that customer called 911, and reported a shot had been fired.

That prompted a major police response, with several officers toting assault rifles as they gathered in the parking lot. Employees packed the parking lot after being evacuated, and nearby roads were clogged with police cars responding to the scene.

Oakbrook Terrace Mayor Tony Ragucci was with officers outside, and said it was all a false alarm.

“We’ve checked all 10 floors, plus a penthouse on the 11th floor. It looks like it’s going to be all clear, and no incident at this time,” he said.

Ragucci estimated 80 officers responded to the scene, and said it was a good training exercise for the department.

He also said the incident speaks to the fears of many Americans in the wake of mass shootings like the one on Monday in Washington, D.C., when a gunman killed 12 people at the Washington Navy Yard. The gunman, former Navy reservist Aaron Alexis, also was killed.

“It just shows that in this time and era, that something like this happens, it’s good to have everybody here. You know what? It’s good to be safe,” Ragucci said.