No passengers, flights or terminals were impacted.
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The building is north of the airport. The fire started on the roof around 7:30 a.m.
One employee told us workers stayed calm.
"They evacuated us and they didn't tell us what it was but I talked to some people and heard it was the HVAC on top of the building. I didn't see anything except a lot of smoke. I didn't see fire. I didn't see anything burning," said United mechanic Scott Pierson.
Officials say the fire appears to have been an accident. They say it started in a heating and air conditioning unit on the roof.
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A spokesperson for United Airlines says this is not impacting their operations at the airport. They are assessing the damage.
Firefighters put the fire out in about an hour.
United Airlines released the following statement:
"This morning, a fire started at a building at our maintenance facility located north of the terminals at San Francisco International Airport. Our employees acted quickly to safely evacuate the building and there are no reported injuries. Acting swiftly, the San Francisco Fire Department was able to extinguish the fire and we are currently assessing the facility. There is no impact to our operations at the airport."