Much easier to tell the IRS 5 MILLION came from Gambling rather than Money Laundering!!
In the absence of any proof otherwise, money laundering is probably the most logical scenario - Arms dealing, drugs?? He was involved in two of the most likely ways to launder money, real estate and gambling. Added to that, he worked for the IRS and also did accounting, so he would know the ropes in hiding paper trails. He owned planes, necessary in either arms dealing or transporting drugs. Granted none of this gives a motive, just explains source of income. Read up on how to launder money through real estate and gambling - far better than through a business where you have to provide detailed receipts of cost doing business that match a IRS profile for that particular type of business.