Because the sender’s account had been registered anonymously, investigators had to use forensic techniques — including a check of what other e-mail accounts had been accessed from the same computer address — to identify who was writing the e-mails.

Eventually they identified Ms. Broadwell as a prime suspect and obtained access to her regular e-mail account. In its in-box, they discovered intimate and sexually explicit e-mails from another account that also was not immediately identifiable. Investigators eventually ascertained that it belonged to Mr. Petraeus and studied the possibility that someone had hacked into Mr. Petraeus’s account or was posing as him to send the explicit messages.

Eventually they determined that Mr. Petraeus had indeed sent the messages to Ms. Broadwell and concluded that the two had had an affair.