Firefighters used ladders to access a parking structure alongside the office building.
At least three people were being treated for smoke inhalation, including two victims who were taken to a local hospital, said Watson.
The fire started in a couple of mechanical rooms on the top of the parking structure, Watson said.
The rooms house HVAC equipment and assorted chemicals. As the structures burned, they released a plume of dark, dirty smoke.
Investigators do not think the cause of the fire is suspicious.
The building, built in 1982, is a downtown office tower with a cafe and health club on site. It has several stories with more than 280,000 square feet of space, according to an Internet real estate page.
The fire started on the top floor of parking garage in a couple of mechanical rooms, according to a witness who works in the building.
"I saw white smoke coming around the corner," said Bob Cosgrove, an attorney.
The smoke suddenly turned black and the building alarm to evacuate went off about the same time, Cosgrove said.
The building was evacuated in a timely and orderly fashion, Cosgrove said.
The fire is out, but firefighters were searching the office building floor-by-floor.
Read more: Smoky fire in downtown Denver Tuesday darkens sky - The Denver Post http://www.denverpost.com/breakingnews/ci_24646328/smoky-fire-downtown-denver-tuesday-darkens-sky#ixzz2mS3tfwYL
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