Politicians keep reminding us not to lose sight of special counsel Robert Mueller’s broader assignment: to investigate Russia’s interference in the 2016 election. If only someone had reminded Mr. Mueller.

One of the biggest failures of the Mueller probe concerns not what was in the final report, but what was not. Close readers will search in vain for any analysis of the central document in this affair: the infamous “dossier.” It’s a stunning omission, given the possibility that the Russians used that collection of reports...