PORTLAND — The cause of a fire that heavily damaged a home at 40 Deering Ave. on Sept. 16 will remain undetermined, Fire Department spokesman Tim Nangle said Sept. 17.

“Investigators have been unable to determine a cause due to the extent of the damage,” Nangle said of the three-alarm blaze that began in the rear of the three-story building.

The fire was reported at about 5 a.m. A family of four escaped without injury; a pet cat died, but another was returned to the family by firefighters.

City Deputy Director of Inspections Jon Rioux said the building will be demolished because of extensive damage.