Firefighters are calling a Wednesday night fire in an abandoned house off Santa Rosa Avenue suspicious, saying it appeared someone had been living there without authorization.

The boarded-up house at the corner of Butterfly Lane, north of East Robles Avenue, was slated for demolition to make way for new development, officials said.

It appeared people had been inside the house at some point before the 8:25 p.m. fire was reported, Rincon Valley and Santa Rosa fire officials said.

Flames were showing from the back of the house when fire crews arrived, Santa Rosa Fire Battalion Chief Mike Jones said. It appeared the fire started inside the house, but the cause had not been determined.

Fire investigators were seeking help from Sonoma County sheriff's investigators, Rincon Valley Fire Prevention Officer Cyndi Foreman said.