LONGMONT, Colo. (CBS4) – A possible hot water heater malfunction sickened two dozen employees at the Oskar Blues restaurant in Longmont Sunday night.

Seventeen people were treated at local hospitals and released. Eight were critically exposed, police said. A decontamination tent was set up at the scene. No patrons were sickened.

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The fire department initially believed a refrigerant leak caused the sickness at Oskar Blues Home Made Liquids & Solids but later shifted focus to a hot water heater.

(credit: CBS)

(credit: CBS)

Employees first noticed a smell and immediately felt sick, the fire department said. Police, firefighters and hazmat crews responded to the restaurant at approximately 12:40 p.m.

“Our best guess at this point is that this was caused by a defective or malfunctioning hot water heater,” a firefighter told CBS4.

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(credit: CBS)

(credit: CBS)

He said it’s possible the hot water heater tried repeatedly to light but was unsuccessful and then sent fumes through a swamp cooler and into the kitchen.

“It’s extremely frightening. It’s very concerning. Their safety is our first concern,” owner Anita Gray said. A sign on the restaurant said: “Closed until further notice. We apologize for this inconvenience.”

The restaurant will be closed until the source can located and repaired.

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*Monday Update: The restaurant planned to be open on Tuesday.