That is what a special counsel does. Trump set himself up when he fired Comey and commented about how he did it because of the Russia investigation.
Special counsels have a broad range of scope. Remember, the ken Starr special investigation of Clinton started out as a real estate investigation and when nothing was found there it evolved into Bill shooting man chowder on Monica's dress. IF Bill had just admitted it, there would have been no crime, but he perjured himself and there were people that were calling for his head.
Trump is the kind of guy that will have all kinds of skeletons in the closet and if Mueller wants to go looking for them, it's within his powers. So the morale of the story kids, don't set things up so you have a special investigator after you.
Sad fact is, if Trump had just left it in Comey's hands you'd probably have Flynn, Manafort and maybe Gates on the hook but the rest of them probably would have gotten off or at the worst Trump would have had a formal censure on his record, but unfortunately Trump has a very poor understanding of basic civics and he dug his own hole.
I wouldn't be surprised that him making the statement on air force one the other day about the payment "you'll need to talk to Cohen, he's my attorney" was the final straw for the FBI to pull the trigger, that FBI team raid was weeks in the making, so right after Trump makes that statement, which works great for a private citizen trying to avoid a question but not for a politician, he may have just sealed Cohens' fate.