Dear Prudence: Two months ago, after posting a picture to my Facebook in which I said I "felt beautiful," I started getting e-mail messages from someone I didn't know that were vicious and cruel.

I am chubby, and my harasser wanted to tell me, as nastily as possible, that I was deluding myself for thinking I could be beautiful. Not knowing who was writing to me, I made my Facebook profile more private.

But recently a friend suggested that I compare the IP addresses of my harasser to the IP addresses of e-mails from people I know. To my shock, I discovered my harasser has the same IP address as my best friend's boyfriend, Adam.

Adam lives alone, and based on the times the messages were sent I do not believe another person sent those messages from his computer. Adam has always been kind to me, and until my discovery I thought he was my friend. His behavior makes no sense, and I don't know what to say to him or to my best friend. What should I do?

Prudence says: I find myself hoping that Adam has a tendency to lose count of his drinks. It's no excuse, but if late at night the Mr. Hyde side of Adam seeks to express itself, then at least there's a proximate cause for his venom.

But if Adam just likes to vent his free-floating hatred under the guise of a false identity, he is a major creep. Just to make sure you weren't going off on a digital tangent, I spoke to some people on Slate's technology team and they said that while identifying an IP address can help point you toward a suspect, keep in mind your evidence is not conclusive. They also suggested that you do as broad a search as possible of the IP addresses of your correspondents to make sure you've turned up a singular match between your tormentor and your friend's boyfriend.

If you remain convinced, I think you should first talk to Adam, then tell your friend. Don't call a solo meeting with him — you want an easy way out if things get even weirder — but next time you're at a social event together, pull him aside and say you'd like a word. Explain that recently you were getting a series of abusive e-mail messages. So you did some investigating and were disturbed to find that the IP address of your new correspondent was the same as his e-mail. Then fall silent and let him respond.

If he indeed is the culprit, let's hope he owns up, abjectly apologizes and says it will never happen again. If so, tell him that obviously you have some thinking to do about your friendship with him, and that this also complicates your friendship with your best friend. Say you are going to tell her, but you will let her hear it first directly from him.

If he denies knowing anything about it, then say while you think the evidence is strong, you accept that it remains a mystery. Add that you're going to let your friend know about your conversation. When you tell her, acknowledge that this is putting her in a difficult situation and that there is an element of doubt, but that you felt you needed to say something.

Take comfort that Robert Louis Stevenson had some pungent observations about the (possible) Adams of the world: "I learned to recognize the thorough and primitive duality of man; I saw that, of the two natures that contended in the field of my consciousness, even if I could rightly be said to be either, it was only because I was radically both."

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