Do not complain to their ISP and get their account closed. You'll destroy evidence.
Now here's what you should do.
You should always take any kind of threat seriously. You should immediately print out the threat and save the email to a disc. Take this in person to your local police station and make a police report. Then follow up with the police. Ask to have an appointment with the detectives assigned to your case . Make sure you follow up.
Unfortunately many police departments will not take an email threat seriously. They will try to parse the wording of the threatening email and attempt to make light of the threat claiming it can be interpreted in different ways. They may just tell you to turn off the computer, change your email and ignore it.
But stil make the report and stay in touch with them. Checking up on the status of the investigation. Try and meet with them in person and put a face to the names on your report.
If they tell you they don't have the resources to try to trace the email and identify the sender they are probably right. Unlike CSI on TV not many police departments have a full time computer crimes division and if they do they are probably overwhelmed with cases.
But don't lose hope. You can hire a private investigator that has the time and resources to locate and identify the sender of the email threat. An investigator that specializes in email investigations has the experience in collecting and preserving the chain of custody of evidence you'll need for court. Many times they will have contacts with local police and prosecutors and can act as your agent in dealing with the police and assist in getting your case the attention it deserves.
Email threats are serious business and should not go ignored. They are clearly sent from someone that does not understand normal social boundaries and they could easily escalate into a real life in person confrontation. The person making the threats will often target the victims family , friends and coworkers if he is not stopped.
If you decide to hire a professional investigator to handle this problem be sure to check him out first. Some state require Pis to be licensed and or bonded. Do an online internet search to see if the PI has done these types of investigations before. Do they have a good reputation or are there many complaints from unhappy customers?
Beware of software companies offering do it yourself "detective software". These softwares are deceptive in claiming they can offer unlimited searches for a one time low fee. This is impossible as many of the searches they claim to provide are only available to licensed professionals.
If a deal sounds too good to be true you can be sure that it's a scam and you should stay away.
