A caller to Rush was guessing that the FBI plant was Carter Page.
My own guess is that the FBI source is a person close to the Clintons who regularly acted as a deep state go-between between the US intelligence community and a circle of retired British ex-intelligence people operating similarly to Fusion GPS who are globalist, anti-Brexit, pro-EU, pro-socialist, etc.
The Brit globalists tried to help Hillary fix the election by mounting an intel op against Trump based on fabricated intelligence.
The claim now by the US IC is probably that if Congress learned about the extent to which the Brits interfered in our election it would jeopardize the special relationship with the UK, so they don’t want to reveal it to Nunes and Gowdy.