A9NEWS viewer reported seeing a bright orange glow and flames shooting through the roof from his apartment two blocks away near 22nd and Stout.
When 9NEWS crews arrived the fire was out, but the scene was very well lit as arson investigators from the Denver Fire Department combed the scene.
Public Information Officer Phil Champagne said they have not determined the cause of the fire but fire investigators believe the fire started outside, behind the building and spread to the inside.
Champagne said there is extensive damage on the second and third floors of the residential structure in the 2400 block of Stout Street, so extensive residents of three units were forced to find another place to spend the night.
Fire crews will likely be in the area througout the day as they continue their investigation.