Two police officers testified yesterday that 20-year-old Navy Seaman Ryan T. Stowers was belligerent toward authorities several times before he was killed by a deputy U.S. Marshal in Rockville last fall. A detective from California told a Montgomery County jury that Seaman Stowers, of Redding, Calif., challenged him to a fight and shouted obscenities at him in June 2003. An officer from Chestertown, Md., said he had a run-in with the sailor about a month before the sailor was killed.