FBI Section Chief Peter Strzok looked dismissive and aloof as he listened to Office of the Intelligence Community Inspector General investigator Frank Rucker relay an alarming lead for the Clinton email investigation on Feb. 18, 2016, at the bureau’s headquarters in Washington, according to Rucker and Jeanette McMillian, an attorney who accompanied him to the meeting. Days earlier, Rucker had discovered an anomaly in all but four of the 30,490 emails Clinton’s lawyers provided to Congress. Each message contained “carterheavyindustries@gmail.com” in the metadata, suggesting that a copy of each message that passed through Hillary Clinton’s private, unauthorized server during her...