1. Hire a lawyer. 
2. Document everything, especially but not limited to communication with your ex-partner and your investors. 
3. Promissory notes may be an issue, but you have to make it clear you were duped, too. 
4. Since the dispute is cross-border, research about your partner and the legislation in his country. (Your American lawyer will not be much help here, but that might not stop them from charging you for sending hollow threats overseas.) Your ex-partner will be a lot less evasive if you have a shot in his home country. However, you should prepare yourself that you won't recover your and your investors' money; cross-border disputes are tricky, and extradition is rarely a working solution. Try to see if he has US assets, but most likely the answer is negative. 
5. Be very careful about what you say to the investors. They might be still in contact with your ex-partner who may be lying to them.
6. Normally characters like this lie that they have no assets. If threatened with jail, however, assets may magically appear.