After one college student shot and killed another, for example, college officials learned the two had exchanged hundreds of e-mails over several months, sometimes several in a single day. As one withdrew from the relationship, the other expressed outrage, disappointment, and increasing desperation. While none of the messages included overt threats, Lion said, their volume and the crescendo of betrayal in the eventual perpetrator’s notes, in retrospect, might have drawn attention to the unhealthy relationship.