You'll need a subpoena in order to get any info from the ISP. We do investigations all the time that almost always hit a roadblock at the end-user's ISP. They are usually very reluctant to even look at the problem until they have a legal document in their hands. About the only time we get lucky is when the other party is another educational institution. They seem to be more willing to help us. At least in your case the ISP is in the US. When you deal with overseas companies, you can forget about it.

Have you reported the incident to abuse@comcast.net? Typically ISPs have completely separate departments to handle security incidents. The general call center techs will be useless in this situation.