SAN FRANCISCO — Eight years ago, Ernie Allen, the head of the International Center for Missing and Exploited Children, called the heads of major banks and credit card companies. Why, he wanted to know, were they letting child pornographers move illicit profits through their systems?

And so began a collaboration between his organization, major banks, credit card companies, Internet service providers, payment processors, and Internet companies like Google and Microsoft. They had hoped to follow the money and quash child pornography for good.

But at some point the money trail went cold. For the last year, Mr. Allen has been working with global law enforcement and financial leaders to find out why.

He may be getting closer to an answer. Today, cybersecurity experts say billions of dollars made from child pornography and illicit sales of things like national secrets and drugs are being moved through anonymous Internet payment systems like Liberty Reserve, the currency exchange whose operators were indicted Tuesday for laundering $6 billion. Preet Bharara, the United States attorney in Manhattan, described it as the largest online money-laundering case in history.