Robert Mueller’s two-year, $25.2 million investigation was supposed to provide the definitive account of Donald Trump, Russia and the 2016 election. Yet even after he issued a 448-page report and testified for five hours before Congress, critical aspects remain unexplained, calling into question the basis for the probe and the decisions of those who conducted it. Time and again in his report and his testimony, Mueller refused to address a wide range of fundamental issues, claiming they were beyond his purview. Some of the issues Mueller and his team did not clarify include whether the FBI...