Howso? If you understand the how the website machinations are made, you can zero in on the death threats.
Every death threat is made from a computer on the internet. These threats can be traced to the IP address of the ISP hosting them. This happens all the time -- a few weeks ago a woman cut a baby out of a pregnant mother (after posting the message on an internet forum). The culprit was traced very shortly because she had posted on a forum via her IP address. Child pornographers are traced to their homes within minutes of their postings.
Strangely enough, only Islamist terrorists seem immune to tracing on the internet.
Watch a (good) homicide detective at work. You'd think he was more interested in the modus operandi of the perpetrator than in the fact that a murder had been committed.