When something similar happened to me I was eventually contacted by the California computer crimes task force, IIRC. Very simple phone call, asking for notes I kept on the situation. Polite.
Then I got looped into the prosecution's long and kind of annoying email chain to everybody involved before there was an eventual going-nowhere of it all. Surprising but that's what happened. So you never know but some basic diligence is typically a good idea. This is not legal advice.
Generally you do a few things.
If something makes you feel uncomfortable, and your agreement allows it, close out the customers account.
Just like facebook / google and friends, I've found it better NOT to get into a lot of back and forth or just point to a generic policy (ie, overseas accounts not supported).
If you need to refund money, make sure you only refund to same payment method. Ie, a credit card refund should not go out by check. I've seen scammers use this with a stolen card, then try and get the refund by check. A few months later card owner contests bill. If you refund back to same card, then when owner protests, the money is already back, nothing to protest.
Consider a hold on funds if you are concerned that they will be returned to issuing entity if you a in the middle on a payment flow. If so you want to make sure your money handling stuff is compliant anyway with KYC and transfer licensing needs.
2. Lawyer up
You might know us from the recent SuisseSecrets (https://www.occrp.org/en/suisse-secrets/) as well as covering russian laundromats through european banks: https://www.occrp.org/en/laundromats/
you can also reach out through jurre[@] occrp dot org
I'd recommend not changing anything about how your app functions until you follow the common advise here. Ask your attorney when you can make code changes. You may be destroying evidence even if it's just "the path they took"
You shouldn't be talking about this publicly either. You could be compromising the future investigation.