Bitcoins cannot be stolen.
Private key can obtained from someone's machine but nobody can claim sole ownership of a private key.
Get tracking coins out of your head. It will never happen.
It's guaranteed to happen in cases like this.
You track the coin and then what? You can't prove anything...
Once they exchange for fiat, you have whatever country they are in investigate them for taxes and sources of income. If they can't prove where the cash to get the bitcoin came from, confiscate the cash. Rinse and repeat.