Bitcoin keys are ECDSA (secp256k1) keys. The same scheme is used in many other areas of computer security; it would be incredibly foolish for the NSA to reveal an exploit they're sitting on, even indirectly.
Would it really be that foolish though? Could it not be perceived as a show of strength? That's why America stole them back in the first place right? $4m isn't a lot to the company hacked or to the USA. Why go to the trouble of stealing them back at all?
