Federal authorities yesterday arrested the fifth and sixth suspects in a massive arson case in Charles County, Md., this month, and law-enforcement sources said other arrests -- stemming from interviews with as many as 16 "acquaintances" -- are likely. The U.S. Attorney's Office said Roy McCann, 22, of Waldorf, Md., and Michael Gilbert, 21, of Fort Washington were arrested yesterday and will be brought before a magistrate in U.S. District Court in Greenbelt today. Authorities have yet to reveal a motive in the Dec. 6 arson that destroyed or damaged 26 unoccupied houses at the upscale Hunters Brooke development in Indian...