Absence of evidence is not evidence of absence. They may have stopped attacks but not revealed any information about this publicly, or engaged in misdirection as to their role using parallel construction.
Absence of evidence is indeed evidence of absence. It's absence of proof that is not proof of absence. It may or may not be strong evidence, depending on how likely evidence is, but it is evidence.
Then, the governing body of the United States, which is to say (nominally, anyway) the people of the United States are utterly unable to determine the effectiveness of this organization under their charge. Close it down.
I, for one, am not aware of any. And that's the real big problem here. They all this sh1t, invade everyone's privacy, and to what ends?
*I am not talking about the NSA spying on non-US citizens.