Writing at The Wall Street Journal, columnist Kimberley A. Strassel asks the question, "What Is The FBI Hiding?" That's behind the WSJ paywall so I'm reacting to regular Scott Johnson's gloss at Power Line. Like Strassel, Johnson takes the Bureau to task for dragging its heels in complying with entirely legal document requests from Congress.
I have a theory about what the Bureau is concealing, based on nothing more concrete than possession of a suspicious mind. I'm wondering if the Bureau had a mole inside the Trump campaign, someone on whom they had dirt and used it to turn that person into a CI.
It is possible the documents the Bureau keeps redacting or withholding would reveal information only available to an insider. Perhaps that insider is still "inside" and providing info, and they're trying to do two things: (1) keep the info flowing, and (2) conceal their probably illegal infiltration of a domestic political campaign. Or, perhaps only (2) is still operational if the mole has since left the Trump team.
Caveat: I am essentially the polar opposite of someone "in the know." I only suggest this possibility because it appears to explain the observed recalcitrance to share documents.