Time was, you’ve got a homicide victim and no suspects, so you ask around. Ask his friends, ask his neighbors . . . Seems to me that today your best bet would be to plug the victims name into a few search engines. Did he post at any Nazi newsgroups? Star Trek newsgroups? The internet could probably tell you a lot.
I know this has been done on some child molesters who’ve killed.
I have seen more than a few investigative journal type shows and more and more bring in the guy’s PC and search e-mails, web content (which is stored offline, so they can ‘revisit’ sites the suspect visited) and other info.
No doubt, the FBI does this.
The smart police personnel, as always, do their homework.
Not all police folks are smart.