You're "assuming" too much.
Because there is no indication (that WE know of) doesn't mean it didn't happen.
I may be wrong but I seem to recall an interview, where Sheriff Arpaio, in answer to a question about a possible indictment said that that getting cooperation from relevant government agencies both state or federal have so far been impossible. Given that, it was his suggestion that that the U.S. Congress was best suited to take up the matter.