A billowing plume from a chemical plant in Conyers, Ga., prompted an evacuation order that affected about 17,000 people on Sunday, caused an hourslong shelter-in-place order for Rockdale County’s remaining 77,000 residents and the shutdown of an eight-mile stretch of the I-20 highway, officials said.

A small fire broke out on the roof of a plant for BioLab, a manufacturer of pool and spa treatment products, at about 5:30 a.m. Sunday, Fire Chief Marian McDaniel of Rockdale County said at an evening news conference.

The fire triggered the plant’s sprinkler system, which caused water to mix with a “water-reactive chemical,” and created a large plume of smoke and chemicals, the chief said.

The authorities previously said the sprinkler system had malfunctioned. The cause of the fire was not immediately clear. There were no immediate reports of injuries.

The fire was contained at first but then reignited around noon, Chief McDaniel said.

“For the past two or three hours, we have been battling that fire to where we now have it out,” Chief McDaniel said.

The water-reactive chemical was still creating a plume and emergency responders were working to move the chemical from the water, Chief McDaniel said. The roof and several of the walls of BioLab’s plant have collapsed.

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