The Village of Oak Lawn is staging what will look like a massive emergency Sunday morning, but is actually a drill.
First responders from over 25 local police and fire departments are taking part.
So will hospital and agency workers on the county, state and federal levels.
The scene will be set to look like an active shooting event.
Participants are being tested on how well they respond to a mass casualty situation.
It starts at 8 a.m. at Oak Lawn Community High School.