In a key finding of the Mueller report, Ukrainian businessman Konstantin Kilimnik, who worked for Trump campaign chairman Paul Manafort, is tied to Russian intelligence. But hundreds of pages of government documents — which Mueller possessed since 2018 — describe Kilimnik as a “sensitive” intelligence source for the U.S. State Department who informed on Ukrainian and Russian matters. Why special counsel Robert Mueller’s team omitted that part of the Kilimnik narrative from their report and related court filings is not known. But the revelation of it comes as the accuracy of Mueller’s Russia conclusions face increased scrutiny. The incomplete portrayal...