Carson City Hall was surrounded by nearly every fire crew in town early Friday after an early-morning worker found what appeared to be smoke filling the building.
She called 911, and fire officials blocked off all the streets around the building shortly before 7 a.m., including Carson Street.
Fire Chief Stacey Giomi said it turned out to be a leaking valve in a rooftop air conditioning unit that spilled Freon gas into the vents.
"They have been doing some work in the HVAC system over there, and we originally thought we had a fire in one of the heating or AC units," he said.
Giomi said the Freon "looked like it was smoke."
Maintenance and fire crews were able to shut down the valve and call air conditioning repair people. Giomi said it took only about 30 minutes to clear the building and open it for city workers.
The problem was fixed quickly and the air conditioning was turned on - which he said was important on what was, thus far, the warmest day of the year.