When the IRS knocks on your door, it is enough to make you shake in your shoes. A visit from a revenue agent (auditor) or revenue officer (collection) who claims to have detected possible fraud means you are already in hot water.
The steps from suspicion to criminal tax investigation have already taken place, and that IS agent already has enough information to prosecute.
It is also possible that you expected this visit if you or your business have been audited recently by the IRS and you have gleaned from your interaction with the service that they may have found something fishy in the handling of your taxes.