Government as a whole. Strictly speaking the CIA, NSA, and FBI but there are totally different security... Strengths? The word is escaping me. NSA being probably the most secretive and the FBI being the least generally speaking, but they do handle different material usually.
I don't think there is a single major agency that isn't 3 letter ignoring the Secret Service tbh. I'm sure they all do to an extent, but it is apart of their job to do so, Particularly Homeland Security
But at the same time they also have a responsibility to protect national secrets, which means that they are directly responsible if a 0 day exploit used against the US surfaces.
Though personally I'm less enthusiastic about software and more about the hardware in terms of exploitation. They are far more difficult to find and can be done more secretivly. Like the Siemens thing. everyone used them. It’s insane how many resources the nsa holds. I wish I could have half the resources they have.
I bet you they have some pretty insane supercomputers under wraps with everything they do. I've read Countdown to Zero Day and always found it more telling about how Govt. handles security than anything. Personally though I think its untrue that they don't care about their own security. If you can't keep secrets then you can't steal them either.