Seattle Fire Department spokesman Kyle Moore said the cause of Monday night's blaze was ruled "incendiary" and started in a detached garage. But they have not determined if there was intent to start a fire.
He said the Seattle police arson/bomb squad was handling the potential criminal investigation.
Either way, for Tim Shields, his house is a total loss.
"I opened the blinds and the garage out back was on fire," he said.
When firefighters arrived, flames were already shooting 40 feet into the air from the home in the 700 block of Warren Avenue North, and the fire was spreading to two adjacent homes.
Residents in all three of the multi-story homes were able to get out safely.
Despite warnings from firefighters about toxic smoke in the air, residents and neighbors gathered on nearby streets as the homes burned.
"I came home to make sure my partner and my neighbor were not inside," Gina Axe said Monday night as firefighters worked to extinguish the fire. "My porch is gone, my dining area and kitchen... wow."
No arrests have been made and officials have not said if any suspects are being sought in connection with the fire.
WATCH: Homeowner, neighbors react to house fire