AUBURN — An early morning fire at an apartment house on Webster Street has been ruled arson, according to a press release from the State Fire Marshal’s Office. The fire was reported at 3:40 a.m.

Auburn firefighters alerted the only occupants of the four-unit building of the fire, and the family of four escaped without injury.

Five Fire Marshal investigators and inspectors are working with Auburn police and fire officials investigating the blaze.

Investigators say the fire started in a vacant first floor apartment to the rear of the building. The family who escaped — two adults and two children — also lived on the first floor in the front.

The apartment house is located at 22-24 Webster Street and sustained heavy damage.