Austin Fire Department officials say the blaze was reported shortly after 5 a.m. Monday. Four children and three adults have been transported to hospitals.
Austin-Travis County Emergency Medical Services spokesman Eric Jakubauskas says two girls, ages 9 and 15, are in critical condition. He says girls ages 15 and 16, plus three adults, have serious injuries.
Officials are trying to determine if the victims are related. Their names weren't immediately released.
Battalion Chief Palmer Buck says the victims apparently were asleep in the small structure when the fire broke out. Authorities are trying to determine what sparked the blaze on a morning when temperatures dipped to the upper 40s.
