Among them is that the Justice Department and Federal Bureau of Investigation outright hid critical information from a congressional investigation. In a Thursday press conference, Speaker Paul Ryan bluntly noted that Intelligence Chairman Devin Nunes’s request for details on this secret source was “wholly appropriate,” “completely within the scope” of the committee’s long-running FBI investigation, and “something that probably should have been answered a while ago.” Translation: The department knew full well it should have turned this material over to congressional investigators last year, but instead deliberately concealed it.

House investigators nonetheless sniffed out a name, and Mr. Nunes in recent weeks issued a letter and a subpoena demanding more details. Deputy Attorney General Rod Rosenstein’s response was to double down—accusing the House of “extortion” and delivering a speech in which he claimed that “declining to open the FBI’s files to review” is a constitutional “duty.” Justice asked the White House to back its stonewall. And it even began spinning that daddy of all superspook arguments—that revealing any detail about this particular asset could result in “loss of human lives.”

This is desperation, and it strongly suggests that whatever is in these files is going to prove very uncomfortable to the FBI.

The bureau already has some explaining to do. Thanks to the Washington Post’s unnamed law-enforcement leakers, we know Mr. Nunes’s request deals with a “top secret intelligence source” of the FBI and CIA, who is a U.S. citizen and who was involved in the Russia collusion probe. When government agencies refer to sources, they mean people who appear to be average citizens but use their profession or contacts to spy for the agency. Ergo, we might take this to mean that the FBI secretly had a person on the payroll who used his or her non-FBI credentials to interact in some capacity with the Trump campaign.

This would amount to spying, and it is hugely disconcerting. It would also be a major escalation from the electronic surveillance we already knew about, which was bad enough. Obama political appointees rampantly “unmasked” Trump campaign officials to monitor their conversations, while the FBI played dirty with its surveillance warrant against Carter Page, failing to tell the Foreign Intelligence Surveillance Court that its supporting information came from the Hillary Clinton campaign. Now we find it may have also been rolling out human intelligence, John Le Carré style, to infiltrate the Trump campaign.

Which would lead to another big question for the FBI: When? The bureau has been doggedly sticking with its story that a tip in July 2016 about the drunken ramblings of George Papadopoulos launched its counterintelligence probe. Still, the players in this affair—the FBI, former Director Jim Comey, the Steele dossier authors—have been suspiciously vague on the key moments leading up to that launch date. When precisely was the Steele dossier delivered to the FBI? When precisely did the Papadopoulos information come in?

And to the point, when precisely was this human source operating? Because if it was prior to that infamous Papadopoulos tip, then the FBI isn’t being straight. It would mean the bureau was spying on the Trump campaign prior to that moment. And that in turn would mean that the FBI had been spurred to act on the basis of something other than a junior campaign aide’s loose lips.

We also know that among the Justice Department’s stated reasons for not complying with the Nunes subpoena was its worry that to do so might damage international relationships. This suggests the “source” may be overseas, have ties to foreign intelligence, or both. That’s notable, given the highly suspicious role foreigners have played in this escapade. It was an Australian diplomat who reported the Papadopoulos conversation. Dossier author Christopher Steele is British, used to work for MI6, and retains ties to that spy agency as well as to a network of former spooks. It was a former British diplomat who tipped off Sen. John McCain to the dossier. How this “top secret” source fits into this puzzle could matter deeply.

I believe I know the name of the informant, but my intelligence sources did not provide it to me and refuse to confirm it. It would therefore be irresponsible to publish it. But what is clear is that we’ve barely scratched the surface of the FBI’s 2016 behavior, and the country will never get the straight story until President Trump moves to declassify everything possible. It’s time to rip off the Band-Aid.

Correction
The FBI briefed House Intelligence Committee members about a top-secret intelligence source but did not allow them to see documents. An earlier version of this article misstated this. https://www.wsj.com/articles/about-that-fbi-source-1525992611