The amount of cryptocurrency flowing into privacy-enhancing mixer services has reached an all-time high this year as funds from wallets belonging to government-sanctioned groups and criminal activity almost doubled, researchers reported on Thursday.

Mixers, also known as tumblers, obfuscate cryptocurrency transactions by creating a disconnect between the funds a user deposits and the funds the user withdraws. To do this, mixers pool funds deposited by large numbers of users and randomly mix them. Each user can withdraw the entire amount deposited, minus a cut for the mixer, but because the coins come from this jumbled pool, it’s harder for blockchain investigators to track precisely where the money went.

Significant money-laundering risk

Some mixers provide additional obfuscation by allowing users to withdraw funds in differing amounts sent to different wallet addresses. Others try to conceal the mixing activity altogether by changing the fee on each transaction or varying the type of deposit address used.