(Portland, OR)  --  Police are opening an arson investigation into an apartment fire that killed a 26-year-old woman in north Portland.  Police say Sarah Oxenreider died in a hospital late last week after being pulled from her burning apartment unit on North Albina Street last Sunday.  Police are now investigating her death as a homicide.  Crime Stoppers of Oregon is offering a 25-hundred-dollar reward for information leading to an arrest.