Fire officials are seeking the cause of a two-alarm blaze in the basement of a house early Friday. One firefighter was injured in a fall on ice while battling the blaze, Fire Chief Michael B. Mansfield said. Smoke was billowing from the structure when firefighters arrived just before 7 a.m., Mansfield said. One firefighter slipped on ice and was hurt, Mansfield said. The injured firefighter was treated at Lawrence General Hospital. The four residents of the house were unharmed, and the fire was brought under control within a half-hour, Mansfield said. The cause is under investigation.