Public safety responders found a 2-year-old girl unconscious in Roxbury Tuesday afternoon, authorities said. Boston police, fire, and Emergency Medical Services workers responding to a report of a cardiac arrest at 1833 Washington St. at 4:41 p.m. found the girl unresponsive, said Officer David Estrada, a police spokesman. EMS took the girl, whose identity was not released, to Boston Medical Center, where she was treated for a life-threatening condition, Estrada said. It was not immediately clear what happened to the child or if foul play is suspected. No other details were available Tuesday night.