BOSTON (AP) – A fast-moving fire swept through a South Boston townhouse overnight, killing two young sisters.

The 3-year-old girl and her 14-year-old sister were on the top floor of their triple-decker at 154 W. 6th St. when the fire broke out just after 3 a.m. on Sunday.

Boston Police spokesman Eddy Chrispin said investigators have not determined whether the three-alarm blaze was caused by arson.

He said no one was in custody by Sunday evening and homicide detectives were combing the site in keeping with the practice of investigating all fire deaths.

Witnesses tell WCVB-TV that the girls’ brother screamed to them to jump out of the burning building. The girls’ mother and brother are hospitalized with non-life threatening injuries.

The fire extended to neighboring homes and the cost is estimated at half a million dollars.

AP-ES-04-06-08 1829EDT