No one was injured when a three-alarm fire swept through the top floor of a three-story apartment building on Munjoy Hill on Monday afternoon, displacing all nine residents.

Firefighters were called to 108 Washington Ave. after 4:30 p.m. when a passer-by saw flames coming from the back of the building and called 911, Division Chief Mike Thompson said.

Only one person, who lives on the first floor, was in the building at the time, Thompson said. The building has four units, including a basement apartment.

The building’s third-floor apartment appeared most heavily damaged. Some other apartments may still be habitable, but fire officials said all nine residents will have to find someplace else to go at least for Monday night.

The first company that arrived sounded the all-hands alarm, and the fire response eventually reached three alarms, drawing 51 Portland firefighters to the scene.

It took about 10 minutes to knock down the bulk of the fire, but firefighters continued to look for hotspots and hidden flames for more than a hour.

Firefighters used hand tools to open up sections of the roof, where fire had crept into the area between the roof and the ceilings, Thompson said.

“There’s a lot of digging,” Thompson said. “There was a wood ceiling with fire above it.”

Thompson and Fire Chief Keith Gautreau said the cause of the fire was not immediately clear, and a fire investigator was en route to begin an investigation into the fire’s origin.

Emmie Michaud, 31, a resident who is displaced, has lived with her partner in the basement apartment for five years.

Michaud said she had stepped out of the building for about 20 minutes when she received word from friends that a building on Washington Avenue was on fire.

“It must have happened really fast, because we were not gone for that long,” Michaud said.