(Portland, OR)  --  Investigators are trying to find out what caused a fire on top of a high-rise building in downtown Portland.  The blaze was reported around 12:30 this morning on the roof of the 15-story Yeon Building, on Southwest 5th Avenue near Alder.  "The Oregonian" reports firefighters had the blaze under control in less than 20 minutes.  No injuries were reported.