Skyway Fire spokesman David Nelson said flames broke out at the Harrington Square shortly before 8 p.m.
The building in the 900 block of Harrington Avenue Northeast was a new construction that was about halfway complete, and was not occupied when fast-moving flames broke out, Nelson said.
"From what I saw, it took a couple of minutes - about two minutes - for the smoke to come up, for the flames to just engulf the rooftop," said area resident Shama Ross.
Thick plumes of black smoke rising above the 117-unit building were seen from miles away all around the Puget Sound area. Multiple agencies responded to the blaze and began protecting nearby structures and evacuating homes.
"When units first arrived, they had heavy smoke on the top floors. Then the wind picked up. Heard explosions - don't know what that means - and it spread really quickly," said Renton Fire Chief David Daniels.
Investigators believe construction chemicals inside the building may have sparked the blasts. The flames did not spread to any other structures, fire officials said.
Firefighters battled the flames defensively as the building was collapsing on itself.
"All of the sudden...three or four stories high, just dropped," said nearby resident george Perantie. "Flames everywhere. the entire building was gone."
The cause of the fire has not been determined. Fire officials said their investigation likely will not start until Wednesday since the remaining structure is clearly unsound.
No one was injured in the fire, but a Renton police officer was hit by a car while managing the crowd near the scene of the fire. The officer was taken to an area hospital with non-life threatening injuries.
The torched building was set to open in January. Another building in the complex, which is scheduled to be completed by August, did not appear to be heavily damaged.
Firefighters from Eastside Fire and Rescue, Bellevue, Kirkland, Tukwila, Burien, Skyway and Kent helped battle the blaze.
These two photos by: Diane Bedlington:
These three photos by: Jason Busch:
Photo By: Elke Peterson: