Suppose a plume of chlorine gas, from parts unknown, creeps over the Douglas County Jail, threatening to poison all of the inmates, visitors and guards inside. It could be chaos, if the authorities don’t have a plan.

As it happens, they do have a plan. Members of the Douglas County Sheriff’s Office and Lawrence-Douglas County Fire Medical practiced evacuating the jail and moving the inmates elsewhere Tuesday morning. It was the first major test of an emergency procedure that could be used in case of fire, hazardous materials or even the admittedly improbable chlorine gas attack imagined in Tuesday’s drill, said Sgt. Steve Lewis, a Douglas County Sheriff’s Office spokesman.

The exercise began at 8 a.m. Tuesday, when sheriff’s deputies at the jail loaded the inmates — actually about two dozen volunteers and local theater actors — into school buses bound for the Douglas County Fairgrounds. The training simulation imagined the inmates, wearing makeup indicating wounds, were to be temporarily housed at the fairgrounds. But part of the drill is that things go wrong when the first school buses arrive there.

Ambulances and firetrucks gathered at the fairgrounds while sheriff’s deputies, called in by radio and carrying red rubber mock firearms, arrived to find a simulated grisly scene: lifeless dummies strewn around an immobile school bus. They checked vital signs, called medics to see to the wounded, and wrapped the inmates disembarking from buses in futuristic silver foil heat blankets.

Actually, the blankets weren’t part of the drill, Lewis said. It was just unseasonably cold Tuesday morning.

The drill, paid for and organized by Douglas County Emergency Management through a Kansas Department of Transportation grant, had been planned for months and is not unlike similar exercises conduced by local agencies every year. But, Lewis said, such preparations were given an added significance after Monday’s terrorist attack in Boston.