North Korea laundered nearly $34 million through a popular cryptocurrency swapping service, security researchers said on Wednesday, as experts raised concerns about authorities’ ability to stay ahead of tech-savvy criminals.

Crypto compliance firm Elliptic announced the findings on Wednesday, alongside an analysis showing that criminals of all stripes have moved at least half a billion dollars worth of stolen funds through RenBridge, a decentralized cryptocurrency bridge, since 2020.

Bridges connect blockchains of different cryptocurrencies, allowing users to exchange one cryptocurrency for another. RenBridge is one such service, and it has become “particularly popular with those seeking to