BRIDGEPORT — A fire of undetermined cause damaged the finished basement of a Red Oak Lane condominium Thursday evening.

A portion of the basement at 351 Red Oak Lane, an end unit, was burned and the entire townhouse-style condo suffered smoke damage, said Assistant Fire Chief John Currivan.

The residents, a couple, were not home when the fire started at 5:30 p.m. They are staying with family, Currivan said.

The fire was extinguished in about 15 minutes, he said.

The assistant chief estimated the damage at $30,000. Firefighters also vented smoke from a neighbor's unit, Currivan said.

The cause of the blaze did not appear suspicious but was under investigation, he said.