Instapundit linked to this article at the Washington Examiner discussing why the FBI hacked Paula Broadwell's account, thus beginning the whole investigation. Basically, Broadwell sent some anonymous harassing emails warning another woman off from getting involved with Petraeus. That woman complained to a friend in the FBI, who did some digging and discovered the anonymous account belonged to Broadwell. They then gained access to Broadwell's regular email account and discovered sexually explicit emails from Petraeus.
A couple things I want to point out since I know some people are upset over the double-standard applied to Petraeus versus the sex-scandals of other politicians. First, Petraeus served in positions in the military and the CIA where, if an enemy had learned of the affair, could have used it to blackmail either Petraeus and/or Broadwell to get information or even alter policy. Second, the time line is not clear to me, but I get the impression that this may have begun while Petraeus was still serving in the military. Affairs are punishable under the Military Code of Justice.
I feel very sorry for Petraeus' and Broadwell's spouses and families. This incident has certainly harmed them. It may end up tearing those families apart. That is the bigger tragedy here. I don't know what reasons Petraeus and Broadwell had in initiating the affair, or what factors may have played a role. Although I know this will get chuckles or a roll of the eyes from some people, one of the miracles in the Judeo-Christian ethos is repentance. It may be that Petraeus and Broadwell cannot continue in their marriages. However, I hope that this causes each of them serious soul searching, enough that they seek repentance, and the forgiveness of both of the families involved and the Lord.