I'm not a lawyer, but I play one on Free Republic...
It seems to me that a Congressional subpoena overrides any NDA that someone may have with the FBI.
The FBI is mentioned nowhere in the Constitution, but Congress is. Congress has plenary power to investigate, and Article VI says that the Constitution is supreme law of the land. That means that the Constitution is supreme over an NDA.
Congress should be able to immunize this person against any penalties from violating the NDA, and let him testify.
-PJ
Jackson is simply looking for a way back to relevance after being shut out of the Obama administration.
He hasn't been the same since his son took the fall with Rod Blagojevich to cover up Obama's involvement in selling his Senate seat in 2008.
-PJ