Fun fact: Bernie Sanders isn't under any investigation at all by anybody. His wife is. He is being questioned with regard to that, nothing more. He is taking a lawyer with him to advise him on what questions he should and should not answer because...
wait for it...
Bernie doesn't have to answer any questions at all that may incriminate his wife in any way as it construes testifying against her, which is prevented by the U.S. Constitution.
That's all there is to it at this point.
Anybody that would EVER go in for questioning with law enforcement without an attorney by their side under ANY circumstances is a patent village idiot.