The IRS is responsible for checking if you are paying the correct amount of taxes, and a component of that is knowing what income you have and where it comes from - since tax depends on the source of income typically (you have to pay different amounts of tax on money you got by selling shares versus money you stole from a shop).

This is how they managed to finally bust Al Capone, and probably other untouchables.

They'll obviously look into how your business has developed over the past years, compare expenses and income, also to other businesses from the same field, and see if something seems odd.

So, starting to book lots of income without scaling expenses etc. will likely raise suspicion and lead to further investigation which can then be passed to law enforcement.

Money laundering can be seen as spending a lot of effort to pay tax on your criminal earnings.

If you managed to launder it in a way that the IRS will accept it and the cops don't notice, you've succeeded and now have spendable money.

Someone who launders money has two ways of handling that on their taxes:

1. lie about it, and commit tax fraud

2. tell the truth about it, and document their laundered money