Risk-free interest? In a bank? Are you kidding?
If you pick a large bank, your deposit is within the limit for federal insurance, I guess you could say it is risk-free.
The FDIC is a joke, they have about 50 B USD, probably in the form of bank deposits. Anyway, they can be locked for months before the resources are released, and probably longer as the state have to intervene, which is not explicitly guaranteed, but you can probably count on it.
For the system as a whole, the question is are the states are solvent.