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"We had [construction] crews on the roof cutting some steel. Embers from that steel dropped on some Styrofoam insulation, and it was that insulation that caught fire," Division Chief Doug Ireland said.

He said the initial 911 call came in at 11:32 a.m. on Thursday. It took his crews 40 minutes to get the fire under control.

Thick, black smoke from the fire could be seen all over the southern part of the metro area. Matthew Bowe works in a building right next door to the unit that caught fire. He grabbed his camera phone and started recording the initial moments.

"The smoke was black and billowing. Eventually it began to smell in our building as well," he said.

There was some initial concern the fire might spread to the next-door Whole Foods, but Ireland said the fire never made it past the wall separating the two units. Ireland said the unit in question was undergoing some partial demolition in an effort to get it ready for a new tenant.

"We just heard somebody say, you know, the 'S' word," Linda McBride said. "And then someone yelled to call 911."

Ireland said one construction worker was treated at the scene for smoke inhalation. The rest of the shops at the Streets of Southglenn remain open.