I'm linking you to the IRS' webpage for reporting suspected tax fraud. There is a telephone number for a Hotline, where you can leave a recorded message. I'd expect that you'd hear from someone shortly. You may even need to compose a letter yourself.
If you are not satisfied by any of the options on this webpage, simply begin by calling 1-800-429-1040. This is the general IRS telephone number. Hopefully you'll find the person you need to report this incident to.
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The Internal Revenue Service has specific procedures in place that involve filing a form to report the type of fraud you believe may be taking place.
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"How do I turn myself in?"
You never E V E R turn yourself in prior to consulting with, and retaining, legal counsel.
You should ALWAYS be represented by an attorney with respect to this type of legal issue.