CHICAGO (CBS) — Police officers, firefighters, and other first responders took part in the largest full-scale active shooter exercise in the state overnight in Rosemont.

Anti-terrorism experts have said these types of large-scale training sessions are critical to prepare for attacks, saying it isn’t a matter of if but when and where the next one is coming.

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The drill in Rosemont involved approximately 300 first responders. The exercise was modeled after a Paris-style terrorist attack, with multiple active shooters. It also included a mock truck bomb detonating at a stadium, and dozens of mock terrorists moving in on a public place.

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Approximately 300 volunteers played the roles of victims and innocent bystanders, who responders needed to help while also searching for the attackers.

“The skills that our first responders are honing tonight are applicable in a variety of circumstances. Part of the process is intended to identify gaps in our preparedness, identify strengths so that we can reinforce our strengths, and fill those gaps wherever they exist,” said Ernest Brown, executive director of the Cook County Department of Homeland Security.

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The drill included first responders from multiple Cook County municipalities, as well as private sector partners.