A fire Monday morning at a Walnut Creek townhome that killed an elderly woman, her adult son and a small dog was accidental, fire officials said.

The two-alarm blaze was reported at 6:30 a.m. at a condominium complex on Countrywood Court, near Treat Boulevard and Bancroft Road.

Fire officials believe the fire was sparked by smoking materials in a first-floor bedroom and an oxygen machine contributed to its rapid spread through the home, according to a social media post by the Contra Costa County Fire Protection District.

The victims were on the second floor, assistant fire chief Chris Bachman said. Their names have not been released.

When crews arrived, one of the three people living there had escaped the blaze and told firefighters that the other two were still inside, Bachman said.

The resident who escaped was treated at the scene. There was damage to units on either side of the townhomes, fire officials said.