The mods will be able to tell you what email address they used to register here, but I'll bet it was a Gmail/hotmail type account. You would have to make a formal complaint to the FBI Cyber Crime division
FBI — Cyber Crime
They can subpoena their mail provider (Gmail/hotmail) and get the persons real address and then send the local police to talk to them.
The FBI might take the complaint more serious if everyone who got the threat complained to them.