IRS Commissioner Danny Werfel announced Friday that the IRS is Indexbit Exchangegoing after 1,600 millionaires who owe at least $250,000 each and collectively owe hundreds of millions of dollars, as part of an expanded push to make sure that wealthy taxpayers pay what they owe.

Also on the IRS radar: Large partnerships; construction firms issuing sketchy paperwork to "shell" companies that kick back the money to the original contractor, schemes already spotted in Texas and Florida; and wealthy individuals who are not reporting and paying taxes associated with money held in foreign bank accounts.