A blaze at 20th and Treat that left 18 people without a home on Friday is being investigated by police as arson.

The San Francisco Police Department said that smoke appeared under the stairwell at 731 Treat Ave. at around 8:28 a.m. Friday after a man, believed to be between the ages of 20 and 40, walked under the stairwell and fled shortly after.

The fire engulfed part of the facade and exterior staircase of the single-family home, causing the fire department to converge on the scene and contain the blaze. While only three people sustained minor injuries, the two families residing in the two-story home have had to relocate for the time being.

Police have not made any arrests.

San Francisco Fire Department spokesman Lt. Jonathan Baxter said the Department of Homelessness and Supportive Housing, the Mayor’s Office of Housing and the Red Cross are working together to find the families housing.