A fire that destroyed a private residence and damaged the Peaks Island post office Monday afternoon has been ruled accidental, and blamed on improperly discarded smoking materials.

Officials from the Portland Fire Department, the state Fire Marshal’s Office and the federal Alcohol, Tobacco and Firearms agency investigated the fire at 76 Island Ave., according to a news release issued Tuesday by Portland Fire Captain John Brennan.

A clerk was working in the post office when the fire broke out at about 1:42 p.m. Monday. There were two tenants in an attached apartment at the rear of the building. The three individuals evacuated the building safely, but a firefighter sustained a back injury.

Two on-duty island emergency responders and six Peaks Island volunteer firefighters were the first to arrive at the fire. They encountered heavy fire at the rear of the building, which contained the two-story family apartment. Firefighters from seven mainland-based fire companies were transported to Peaks Island by city fireboats.

“When we arrived on the island, the Peaks Island firefighters had the bulk of the fire knocked down,” Deputy Chief Kevin McGuire said.

While the apartment was destroyed, the post office was not burned, but did sustain significant smoke damage. All of the mail stored in the post office at the time was recovered undamaged. The seasonal occupants of the apartment have found other housing.

“The building did sustain substantial (smoke) damage,” Steve Doherty, the spokesman for the United States Postal Service, said in an email. “Street delivery will continue uninterrupted with the carriers dispatching from a different location. A mobile post office will be in place to handle retail operations as plans are finalized for a temporary alternative location.”

Doherty said the space used by the Postal Service was rented.