Crews from Snohomish County Fire District 7 were dispatched to the fire just before 12:30 a.m. in the 1100 block of 156th Place SE.
A neighbor called 911 after spotting flames inside a first-floor window, said Autumn Waite, a fire district spokeswoman.
The neighbor and his son quickly knocked on doors to clear the three-story apartment building. They also used a fire extinguisher on the flames, Waite said.
Crews were able to finish putting out the fire minutes later.
Damage was contained to one apartment, which was unoccupied at the time of the fire. One person was treated for minor smoke inhalation and taken to a local hospital as a precaution.
More than 20 firefighters responded from fire districts 1 and 7.
The cause of the fire remains under investigation.
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