According to US law they can arrest people for money laundering even if the money was laundered outside of the US.
Here's Russian opposition leader Alexei Navalny's old statement:
The West plays along. Countries like Switzerland or Britain offer a safe haven. Although all these countries have laid down rules to fight international corruption, the reality looks slightly different. For years now I’ve been trying to make them cooperate: the FBI, public prosecutors in Germany and Switzerland, the Serious Fraud Office in the UK... All in vain. The European media complains a lot about the Russian mafia and corruption. I keep telling them "open up a case — at least one!" But nothing ever happens.