The proposal looks enticing, frozen there on the computer screen. “Bored?” it reads. “Let’s have some fun.” The message apparently comes from a 19-year-old student at the University of Nevada, Las Vegas. She wants to party with a “cool guy.” She is “definately” looking to hook up today. Her come-on is in the form of an ad posted on the “casual encounters” section of Craigslist, a dark corner of the online classifieds network that’s devoted to no-strings-attached liaisons. “Send me a message and we can trade pics,” she continues. “I am for real, and want this to be discreet. My father’s well known in Vegas.” This last touch is nice. The rebellious child of a washed-up pop performer? Some sex-crazed refugee from the Wynn dynasty?

Actually, no. The author’s real name is Michael—as in, not a chick. Michael Crook is 28, not 19. He has greasy dark-brown hair and a skin condition. He’s not studying at UNLV; in fact, he’s not even in Vegas. He’s sitting at a broken-down desk in Syracuse, New York, in a tiny room with nothing on the walls, junk cluttering the floor, and a jar of off-brand VapoRub and a losing scratch-off lottery ticket lying next to the computer monitor. It’s a cold, gray autumn afternoon, and Crook’s doing what he likes to call a sting. So far he’s hit up 14 American cities with fake ads like this one. He checks the local weather to make them more realistic (“Dreary outside, so let’s hook up!”). Then he sits back and watches the responses pour in, literally hundreds within hours: respondents freely offering phone numbers, e-mail addresses, and pictures, including cock shots.

And then Crook busts them. In September he posted an ad, mined all the personal information he could from the responses, and put everything on his site, (He says he got more than a million hits in just two weeks, until one of Crook’s victims had him served with an injunction—and Craigslist, citing copyright infringement, demanded that he shut down the site.) But he didn’t stop there. Once his prey was caught, he escalated to torture. Take the married Las Vegas man who sent Crook a photograph and phone number: Crook followed up by sending e-mails about the exchange to the man’s wife and coworkers and, for good measure, the media. “I have never been humiliated in quite this fashion,” the man told the Las Vegas Sun in October. “The nights without sleeping . . . it’s just been unreal.”

Tough titty, says Crook. “I’ve had guys beg me, ‘Oh, no, please don’t expose me—my wife will catch me.’ Blah blah blah. She needs to know.”

In an era of Dateline pedophile stings and watch-’em-pay jubilance, a sex-baiter like Crook represents a new breed: the Internet vigilante. Crook is a self-appointed marshal of the cyberfrontier, a private investigator who’s hired himself not to expose criminals—the morally dubious actions of his quarry aren’t illegal, after all—but to halt whatever behavior he deems deplorable. After one guy used his corporate e-mail address to invite the ad’s “girl” to his conference room for a little mischief, Crook notified the respondent’s boss. In a similar situation involving another company, he faxed a copy of the compromising e-mail to the sender’s manager.