U.S. law enforcement calls it a loophole big enough to throw a desktop through, and about as frustrating.
But some free speech advocates say the issue is overblown, because no actual crime is being committed, at least according to U.S. law.
They're talking about "virtual child porn," the computer depiction of adults having sex with children. It has found a home at places like the popular Second Life game, which is available online.
Many child advocates see it as the most troubling manifestation of child sex abuse that's come along in years.
And they blame a 2002 court ruling that tossed out a law that made it illegal.
Last week authorities in Germany — where virtual child porn is a crime — launched an aggressive investigation to track down anonymous video game users who created virtual child porn on Second Life.
That wouldn't have been the case in the United States, where in 2002 the U.S. Supreme Court ruled that large sections of the 1996 Child Pornography Prevention Act, including the depiction of virtual child porn, were overly broad and unconstitutional. Free speech advocates and pornographers had challenged the legality of the act, and six of the nine justices sided with them.
Then-U.S. Attorney General John Ashcroft was furious, saying the ruling would make it "immeasurably more difficult" to investigate and prosecute pedophiles and child pornographers. In an unusually blunt public rebuke to the court, the Justice Department ordered that all pending child porn cases be reviewed to see whether the defendants could be prosecuted under broader obscenity laws.
"It's very disturbing for child advocates because it's sort of [a] loophole," said Brad Garrett, a former FBI agent and an ABC News consultant.
Another agent, who worked for the FBI's Innocent Images Task Force, told ABC News he was "devastated" by the ruling.
"I still can't can't believe it," said the former agent, who declined to be identified because he is not authorized to talk about his work with the agency. "All virtual porn does is satisfy [pedophiles] until they can find their next victim. It feeds their addiction."
As computer technology gallops rapidly into uncharted territory, interactive, or "open source," video games have allowed some users to break or subvert gaming rules and create virtual child porn — the virtual depiction of an adult having sex with a child.
"There are increasing numbers of cases in which people charged with child porn related offenses are arguing … that the children in the images aren't real," said Ernie Allen, president of the National Center for Missing and Exploited Children. Working closely with law enforcement agencies nationwide, Allen's agency has for years been at the forefront of efforts to track and catch pedophiles.
"We believe that the kinds of images included in 'age play,'" cyberspeak for online sexual fantasies with minors and young people, "are an increasing phenomenon and will be a growing concern," Allen said.
But like many others who spoke to ABC News about the vexing problem virtual child porn poses to U.S. law enforcement, Allen was realistic.
"The first and primary focus is protecting real children against this horrible abuse," he said, "but this is a problem that is coming."
"Second Life is a place where anyone can have just that," Michael Fitzgerald wrote in February on Inc.com, a Web site for entrepreneurs. "There are minutely detailed replicas of Rockefeller Center and human-size raccoons; sex and sadism and spiritual retreats; conference calls and a currency exchange. Almost all of it is created by the people who pay to dwell in it. Linden Lab, the San Francisco company that created and owns Second Life, acts as a sort of laissez-faire government. It makes money primarily by selling property, of which it can conjure an infinite amount."
An online user with the name Stephen Northport described the game as "tomorrow's playground."
But that utopian world has been invaded by pedophiles, German police told ABC News.
"A player came to us to report that he was invited to virtual child pornography meetings," said Peter Vogt, chief prosecutor from Germany's central office. "Child pornography is a punishable crime in Germany, however it is happening, and so is virtual child pornography, which is punishable by up to five years in prison."
Vogt, a widely recognized expert on child pornography investigations, said the game company, Linden Labs, had been cooperating with authorities. Linden Labs' legal counsel, Ginsu Yoon, confirmed this to ABC News.
Vogt's office was contacted by a German television station, whose reporter, a Second Life player, was invited to attend the virtual child porn meetings, and German authorities sprung into action. They believe the user is a German because he communicated in that language.
Lawrence Walters, a lawyer who advises online casinos and gaming companies, said that the big publishers of online games were "very hesitant to allow online users too much freedom because these issues come up. You have to expect just about anything in real life to occur on Second Life."
Brenda Brathwaite is a professor of game design at Savannah College of Art & Design. Her book "Sex in Video Games" was published in the fall.
"It's an anomaly," she told ABC News via e-mail. "I've come across very, very few examples of it. The few I have come across involve two consenting adults on an adults-only server engaged in role play [i.e. 'dress up like a schoolgirl']. The much greater threat … is virtual spaces where actual adults and children connect, like on IMs [instant messaging] or via numerous social networking sites."
Still, Walters acknowledges, the dawn of virtual child porn is upon us.
"The marketplace is clearly demanding more open-source interactive capability," he said. "Users are becoming more and more technically savvy. The new generation is demanding that they be able to program games as they desire. As they become older and begin being more of a buying power, they are going to demand more open sourcing. This is only going to become a bigger and bigger issue."
Longtime child abuse victim advocate Judy Cornett of Florida gets angry when she discusses the legality of virtual child porn.
"Do you mean to tell me that after Joe Blow watches child porn, whether it is virtual or otherwise, that he is not going to look at a real child with the same sick thoughts? What if he sees his niece within the next few minutes, or his daughter who is the same age? There is not a button to press to stop these feelings. Fantasy play has an enormous role in the development of compulsive rapists or child molesters. The Internet is a hunting ground for predators today," she said. "And virtual child porn just makes it a legal hunt."