The Internal Revenue Service has sent letters to over 10,000 cryptocurrency holders warning them that they may be breaking federal tax laws.

The IRS has been sending out three different letters to American taxpayers, depending on the information the IRS has on the recipient. Thousands of Americans, and many more people around the world, hold cryptocurrencies as either an alternative currency or for speculation. For the most part, governments haven’t been able to regulate cryptocurrencies very efficiently, due to their nature. Cryptocurrencies are encrypted lines of code stored on a server called a blockchain, as opposed to fiat currencies which are held in central banks and regulated by national governments.

The move by the IRS comes as crypto makes a comeback. After a shocking skyrocketing in value in January of 2018, crypto became less popular as the year went on. Bitcoin finished the year worth under $4000, but it’s come back in the summer of 2019, topping $12,000 mid-July. Now with the help of several large retailers and Facebook’s Libra, cryptocurrencies are entering the mainstream again. At the same time, the popularity of cryptocurrencies among terrorists, drug dealers, and many other bad actors has kept regulatory concerns high.

If you’re wondering how the IRS even figured out who to send letters to, there are few explanations. One possibility is the information on over 13,000 Coinbase accounts provided to the IRS in 2018 under a federal court order. Coinbase handed over data on customers who bought, sold, sent, and received amounts worth over $20,000.