If they did something wrong, or we think they may have done something wrong, they should be investigated, and prosecuted if evidence supporting criminal charges are discovered. It does not matter who they are.
We are currently investigating Trump, spending millions of dollars, all for something that is not even an actual crime in America. There is no criminal statute that makes “collusion” with Russia to help him win the election a crime even if true.
So if we can expend all this time and money investigating things that are not even crimes, we can certainly investigate things that are crimes.