There’s no dress code whatsoever. We guess that this should be guided by common rules of decency. Although Americans are very relaxed about what they wear, We most certainly wouldn’t recommend shorts, open toes, too much visible skin, etc. Also, don’t even think about very bright, neon color, or T-shirts with messages.
We would also think twice before coming for an interview dressed as skinhead, punker, or something similar. The best advice – dress as you would for any business interview in a decent company, or as if you were working in any State agency.
Don’t worry too much, visual impression, although relatively important, is not the door opener (although probably can close some doors). Rather think of what you want to say, how you want to present yourself during the interview. This is much more important and it should be the main point of your interest.