“What did the president know, and when did he know it?” On June 28, 1973, during a hearing before the Senate Watergate Committee, the ranking member, Tennessee Republican Howard Baker, posed the central question of the investigation.
Forty-five summers after Baker uttered those historic words, the same question now hangs over the alleged collusion between Donald Trump and Russia in the 2016 election. But this time the question is directed at the investigators. It runs like this: What did the FBI know, when did it know it—and...