BURLINGTON — Vermont fire investigators say an early morning fire in a vacant Burlington apartment was arson.

While the apartment on Ethan Allen Parkway was vacant, three other apartments in the same building were occupied when one tenant called 911 at 5:15 a.m. Tuesday after hearing a smoke alarm go off in the empty apartment.

The families in the three apartments left the building, but they were allowed to return a short time later after firefighters extinguished the fire.

Investigators, along with a police dog, investigated and determined the fire was deliberately set.