President Donald Trump and his supporters are circulating an explosive theory: The FBI, they say, may have planted a mole, or "spy," inside the 2016 campaign to bring him down. The unverified allegation has lit up conservative media and earned space on Trump's Twitter feed just as special counsel Robert Mueller enters his second year in the Russia probe. But where did the allegation come from? Like many conspiracy theories, it appeared to grow out of a less sensational truth: U.S. surveillance on foreign officials - a common practice in the world of spycraft - likely picked up what's called...