The United Kingdom's National Crime Agency (NCA) seeks to regulate the crypto coin mixers under the country's laws against money laundering.
Coin mixing tools are popular in the decentralized world as they maintain the privacy of transactions. These tools often mix several transactions to obscure the origin of a particular transaction. Then the recipient receives the transactions from a mixing "black box" comprised of hundreds of transactions from various wallets. While privacy-focused, these tools often face regulators' ire as they are also a known way for hackers and criminals to wash their funds.