(Gresham, OR)  --  Gresham police are searching for a missing Washington man who was in town to participate in a swimming competition.  Authorities say 69-year-old Thomas Walker of Sammamish left Mount Hood Community College around eight o'clock yesterday morning and hasn't been heard from since.  Family members are concerned about Walker's heart condition.  Walker is described as white, five-feet-eleven, 180 pounds, with gray hair, was last seen wearing a red t-shirt and swim trunks and carrying a black athletic bag.