The collapsed houses - one clad in powder-blue siding, the other in white - were on the north side of Spruce Street near 60th Street. The cause of the collapse, which occurred around 11:45 a.m., remained under investigation. It appeared to have happened along the party wall between 6015 and 6017 Spruce.
L&I Commissioner Carlton Williams said the investigation could take days. Investigators do not know what part of the wall - which stretched from the homes' basements to the second floors - caved in, he said.
Williams said inspectors would check for "cracks in the foundation of the party wall, particularly in the lower part of the basements" inside the homes. They will also look for irregularities where the wall curves outward.
Lives could have been lost if people had been inside, Williams said.
He called a collapse of homes where people live "unlike other properties we normally deal with," contrasting it with the caving-in of vacant structures.
Usually, collapsed structures are empty and break down over time due to the elements, he said.
He advised property owners to routinely inspect their homes, adding that if people are uncertain about their house's safety, they should call the department, which will send an inspector out.
Christine Chapman, 62, who has lived for 30 years at 6017 Spruce, was most concerned about freeing her trapped dog, Brie, a Scottish deerhound.
Her son, Daniel, called her after a neighbor called him about the collapse. Christine Chapman was at work and came right over.
She was eventually permitted to enter the home and search with Jen Leary of the Red Paw Emergency Relief Team, a nonprofit that helps rescue animals in disasters.
Leary went in first. She could hear Brie, but didn't see her. When Chapman entered and began calling, Brie immediately responded to her voice.
They coaxed the dog to cross the dining room and were able to get her out, Leary said.
Exiting with Brie, Chapman flashed a thumbs up. Onlookers responded with cheers.
L&I declared both collapsed properties "imminently dangerous." Williams said the city would begin demolition after contractors hired by L&I remove as many of the residents' personal items as they safely can.