All of this also raises questions about Special Counsel Robert Mueller’s investigation. The Fusion news means the FBI’s role in Russia’s election interference must now be investigated—even as the FBI and Justice insist that Mr. Mueller’s probe prevents them from cooperating with Congressional investigators.

Mr. Mueller is a former FBI director, and for years he worked closely with Mr. Comey. It is no slur against Mr. Mueller’s integrity to say that he lacks the critical distance to conduct a credible probe of the bureau he ran for a dozen years. He could best serve the country by resigning to prevent further political turmoil over that conflict of interest.

The American public deserves a full accounting of the scope and nature of Russian meddling in American democracy, and that means following the trail of the Steele dossier as much as it does the meetings of Trump campaign officials.