There are special division for the police department where you can complain things like this and it will be an advantage for a country that recognize bitcoin and cryptocurrencies.

FBI can do the job if you're living in America but if not try to search on what particular agency is assigned to receive these type of complains.

And also mate, don't underestimate the bravery and knowledge of police force. I do support their arm for controlling and reducing the crime rate and these such acts will be solved by them.