Who can I hire to prove the source of an online death threat?

Question

I know who sent it, have date, time and screenshot. It was sent through a site called PrivNote 

Answer 1

The police do have computer programmers who can go through that.

Answer 2

Contact the government IG!  Online anonymity is a Constitutional right, so unless they have a warrant they can't doxx somebody.

Answer 3

The first thing that you need to do is to contact the sheriff's department and take a screenshot! The sheriff has the legal power to obtain the IP address of the person that sent the threat

Answer 4

Try the police, they can look into it