James Comey gave his follow-up testimony to Congress this week, in which he continued to profess memory loss about most of the 2016 investigation of the Trump campaign. Then again, the joke was on the former director of the Federal Bureau of Investigation. Thanks to two years of dogged investigation, House Republicans already know the story of the FBI’s 2016 doings. Mr. Comey wasn’t there to provide new details. He was there to account for his actions.

That’s the crowning achievement of this 115th Congress. Tax and criminal-justice reform and judicial appointments are all hugely important. But House and Senate investigators get pride of place for unraveling one of the greatest dirty tricks of our political times, in which a Democratic administration, party and presidential campaign either co-opted or fooled the FBI into investigating the Republican campaign. Lawmakers got to the bottom of this despite partisan attacks and institutional obstruction. Congress has taken that probe about as far as was ever going to be possible. The next steps are up to the White House.

In January 2017, CNN reported the explosive news that “classified documents” from a “credible” “former British intelligence operative” alleged collusion between the Trump campaign and Russians. It sounded bad and set off a hysteria that led to the recusal of Attorney General Jeff Sessions, the firing of national security adviser Mike Flynn, the launching of half a dozen investigations, and the appointment of special counsel Robert Mueller. Senior officials, including Mr. Comey, watched all this in full knowledge of the dossier’s provenance. They said nothing.

It was left to the House Intelligence Committee, under Chairman Devin Nunes, to extract the real story: that the Democratic National Committee and Hillary Clinton’s campaign hired opposition-research firm Fusion GPS, which in turn retained a British gun-for-hire (Christopher Steele) to compile the so-called dossier; that Fusion injected this into the FBI, the Justice Department and the State Department; that this political dirt was a part of the FBI’s decision to launch an unprecedented counterintelligence investigation (which included human informants) into a presidential campaign; that this dirt was also the basis for a surveillance warrant against former Trump aide Carter Page; that the “credible” Mr. Steele was fired by the FBI; and that the FBI withheld the most sordid details from the Foreign Intelligence Surveillance Court, which granted said warrant. And we separately know the Obama administration was engaged in the unmasking of U.S. citizens and leaking of classified information.

Congressional Republicans have the names, the actions, a timeline and the documents. The main elements are all there, and it’s thorough. Investigators tell me the only major open question is the role of Joseph Mifsud, the mysterious Maltese academic who approached then-Trump adviser George Papadopoulos in the spring of 2016. It’s unclear who, if anyone, Mr. Mifsud was allied with in that venture—the feds, the oppo players, former or current British intelligence? Congressional investigators were unable to track him down for an interview.

Yet the public doesn’t have this full story yet—because Congress isn’t authorized to tell the key remaining parts, which are classified. According to congressional Republicans, those documents—FBI interview summaries, the Page warrant application, internal Justice Department communications that explain who knew what and when—contain even more explosive revelations. More importantly, they will help answer the question of whether anyone broke laws, for example by lying to a federal court or to Congress. Some Republicans believe the release would be sufficient to justify a Justice Department investigation.

Which is why the interviews a joint House Judiciary and Oversight task force has been holding since summer are so important. The task force has been working its way through key players—former FBI agent Peter Strzok, former FBI counsel James Baker, Justice Department official Bruce Ohr, former Attorney General Loretta Lynch, Mr. Comey and others. (One pity is that the task force didn’t make it through more of the 40 names Mr. Nunes sent it.) But these interviews haven’t necessarily been about eliciting new information. They’ve been as much about getting a formal accounting from the central actors, claims that can be held up against the information when it is finally released.

That also explains why House investigators haven’t released an official report on their findings. They know the story, but they can’t tell it comprehensively until President Trump follows through and declassifies the relevant documents. The president is said to be waiting for the Mueller probe and prosecutions to end. But why? This is about transparency. The president also owes it to these lawmakers, who began the hard work of investigating the 2016 election, to finally allow them to finish it.

Write to kim@wsj.com.