The most notable aspect of the Mueller report was always what it omitted: the origins of this mess. Christopher Steele’s dossier was central to the Federal Bureau of Investigation’s probe, the basis of many of the claims of conspiracy between the Trump campaign and Russia. Yet the Mueller authors studiously wrote around the dossier, mentioning it only in perfunctory terms. The report ignored Mr. Steele’s paymaster, Fusion GPS, and its own ties to Russians. It also ignored Fusion’s paymaster, the Clinton campaign, and the ugly politics behind the dossier hit job.

Mr. Mueller’s testimony this week put to rest any doubt that this sheltering was deliberate. In his opening statement he declared that he would not “address questions about the opening of the FBI’s Russia investigation, which occurred months before my appointment, or matters related to the so-called Steele Dossier.” The purpose of those omissions was obvious, as those two areas go to the heart of why the nation has been forced to endure years of collusion fantasy.

Mr. Mueller claimed he couldn’t answer questions about the dossier because it “predated” his tenure and is the subject of a Justice Department investigation. These excuses are disingenuous. Nearly everything Mr. Mueller investigated predated his tenure, and there’s no reason the Justice Department probe bars Mr. Mueller from providing a straightforward, factual account of his team’s handling of the dossier.

If anything, Mr. Mueller had an obligation to answer those questions, since they go to the central failing of his own probe. As Florida Rep. Matt Gaetz asked Mr. Mueller, how could a special-counsel investigation into “Russia’s interference” have any credibility if it failed to look into whether the Steele dossier was itself disinformation from Moscow? Mr. Steele acknowledges that senior Russian officials were the source of his dossier’s claims of an “extensive conspiracy.” Given that no such conspiracy actually existed, Mr. Gaetz asked: “Did Russians really tell that to Christopher Steele, or did he just make it up and was he lying to the FBI?”