AMHERST - Firefighters are continuing to investigate the cause of a Saturday night fire at a North Amherst apartment complex, though the interim fire chief said it is possible a determination may never be made.

Interim Fire Chief Lindsay Stromgren said Monday that the fire at Presidential Apartments, at 950 North Pleasant St., started in a couch inside apartment 54. The couch was vaporized and the resulting heat and smoke caused extensive damage throughout the unit, which has been declared uninhabitable by the building inspector.

But how the couch caught on fire remains unknown.

"Our investigators are still interviewing the tenants," Stromgren said.

If they are unable to provide any further information, the case will be closed. "We're at a dead end," Stromgren said.

No one was in the apartment when the fire started, and firefighters were called at 9:58 p.m. If the tenants had been home, firefighters might have gotten there earlier and damage might have been more limited, Stromgren said.

Before it is habitable, the unit will have to be gutted and completely renovated, Stromgren said. Three other units sustained more minor damage and will need some limited renovations, including replacing smoke detectors in the common areas and installing new windows which were smashed out by firefighters to ventilate the building.

The building itself didn't sustain any structural damage as the fire was contained to the single unit. Stromgren said Sheetrock effectively contained the fire.