Firefighters say illegal fireworks ignited vegetation, and the blaze spread to a southeast Portland home.
The fire department says witnesses saw someone light what was described as a "mortar" round and toss it on the ground near bushes outside the house.
Crews found flames in the attic when they arrived late Thursday at the duplex in the Sellwood neighborhood. The fire downed a power line, complicating the firefighting effort.
The people in the house and their pets were safely evacuated.
There was no immediate damage estimate.