People who have the net worth and income like the Trumps are pretty much under constant audit. If there was something to find we would have seen it years ago. This is harassment plain and simple.
Given that conservatives go in the DOJ express lane for prosecution, they absolutely would have charged Trump in 2015 or 16 if a crime was there. If they had anything at all he would have had Koskinen leak it.
Trump has been under constant investigation and surveillance since they got wind that he was going to run. It probably started the first time he publicly criticized 0bama. Years and years of investigations; by now centuries of manpower wasted looking for any dirt at all, out of which came absolutely nothing.