President Trump accused the F.B.I. on Friday, without evidence, of sending a spy to secretly infiltrate his 2016 campaign “for political purposes” even before the bureau had any inkling of the “phony Russia hoax.” In fact, F.B.I. agents sent an informant to talk to two campaign advisers only after they received evidence that the pair had suspicious contacts linked to Russia during the campaign. The informant, an American academic who teaches in Britain, made contact late that summer with one campaign adviser, George Papadopoulos, according to people familiar with the matter. He also met repeatedly in the ensuing months with...