IRS criminal investigations are rare. The Criminal Investigation division pursues roughly 2,000 cases per year. They focus on willful fraud with large dollar amounts, affirmative concealment, and badges of fraud. The typical taxpayer who fell behind on payments is not their target.
If you receive a target letter or learn you are under investigation, stop talking immediately. Do not speak to the IRS, do not speak to your CPA about it, do not speak to anyone except a criminal tax defense attorney. Attorney-client privilege is your only protection. This requires immediate legal counsel.