If the IRS CID (Internal Revenue Service Criminal Investigation Division) came to talk to you the has been investigated thoroughly. The agents have done their homework and are hoping that you will cooperate. Whether cooperation or an assertion of your right against self-incrimination is the best path is dependent on many factors. Any belief that you can "talk your way" out of this is foolish and may harm you. Remember that giving a false statement to the agent is a separate offense.
Consult with an experienced federal criminal defense attorney BEFORE you make any statements. The attorney should have specific experience with Tax preparer issues.
Of course, every answer or response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. This answer/response should NOT be relied upon to make any legal decisions. Seek the advice of an experienced Federal and/or state criminal defense attorney in your jurisdiction BEFORE you say or do anything.
Discuss the matter with a criminal defense lawyer before calling the IRS.
I am a former federal and State prosecutor and have been doing criminal defense work for over 17 years. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012 and 2013. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. Martindale-Hubbell has given me its highest rating - AV Preeminent - in the areas of Criminal Law, Personal Injury, and Litigation. According to Martindale-Hubbell”AV Preeminent is a significant rating accomplishment - a testament to the fact that a lawyer's peers rank him or her at the highest level of professional excellence." Fewer than 8% of attorneys achieve an AV Preeminent rating. I also have the highest ranking – “superb” – on Avvo. The above answer, and any follow up comments or emails is for informational purposes only and not meant as legal advice.
Obtain a lawyer who does criminal tax defense. Do not consult with an accountant nor speak to anyone about this matter. Anything you say, except to a lawyer who is representing you can be used against you.