But Trump and his backers are wrong about what it means that the FBI reportedly was using a confidential source to gather information early in its investigation of possible campaign ties to Russia. The investigation started out as a counterintelligence probe, not a criminal one. And relying on a covert source rather than a more intrusive method of gathering information suggests that the FBI may have been acting cautiously — perhaps too cautiously — to protect the campaign, not undermine it. As a former FBI counterintelligence agent, I know what Trump apparently does not: Counterintelligence investigations have a different purpose...