I think of this like ladder. Arrest the first rung. Some od them will give you the next rung and eventually you have witnesses against the top rung. That’s the way they went after the mafia after Valachi started singing and set a precident.
This is also a gradual way of getting to the top culprits which gets the nation used to the idea that there is something very wrong going on and so when they finally get to the corrupt people at the very top like Hillary and Obama, people will not be in such a state of shock as they would have been if the news was broken too suddenly.