Ultimately, the Congress is privy only to what the adminstration wants to make them privy to. Congress doesn’t have its own intelligence service. It has to rely on the CIA, NSA, FBI, and DIA to provide it with information. If those services decide or are directed, quietly, to hold material back, Congress is left with few realistic alternatives, short of issuing subpoenas for the information. And if they don’t even know what information is out there, they can’t very well issue a subpoena for it.