Bitcoin is often described as an anonymous way to pay online, but a group of researchers says that data revealed through the cryptocurrency might help track down sex traffickers.

In a paper presented last month at the Association for Computing Machinery’s annual conference on knowledge discovery and data mining, researchers analyzed sex-related ads placed on the controversial classified listings site Backpage. They sought to locate groups of ads placed on the site by the same person, which they say could help investigators spot behavior that points to human trafficking and sex slavery.

“Such a tool would allow officers to confidently use timing and location information to distinguish between ads posted by women voluntarily in this industry versus those by women and children forcibly tracked,” they write. “For example, groups of ads—posted by the same owner—that advertise multiple different women across multiple different states at a high ad output rate, is a strong indicator of trafficking.”

One technique, using statistical analysis to compare the actual texts of different ads, proved to have a high success rate at finding postings apparently by the same group, says coauthor Rebecca S. Portnoff, a graduate student in computer science at the University of California, Berkeley. To validate whether their algorithms were, in fact, finding ads written by the same people, they looked for postings featuring common email addresses or phone numbers. That’s a technique often used by law enforcement and other investigators looking for sex or labor trafficking rings. Portnoff says it has limitations, since it’s easy to register new phone numbers and email accounts, but for now, it’s the most reliable method available.

“The best ground truth that exists in the space is the hard identifier, the phone numbers, and email addresses,” says Portnoff, who is also a research and data scientist at Thorn, a nonprofit cofounded by actor Ashton Kutcher to fight child sex exploitation.

The researchers also looked to the public, shared ledger of bitcoin transactions to find payments for multiple ads that seemed to stretch back to the same bitcoin wallet, essentially the cryptocurrency’s equivalent of a bank account.

Backpage has come under fire over the past few years for hosting sex-related ads. While the site looks and functions almost identically to Craigslist, the company has often been reluctant to stamp out erotic listings, and the postings in many cities where Backpage operates are dominated by ads for sexual services. That’s led Visa and Mastercard to cut ties with the company. (While Backpage eliminated some adult-themed classified sections early this year, sex ads have generally migrated to other sections, especially personals sections nominally focused on traditional dating.) The lack of credit card support has left the sex-ad posters paying for prominent placement with limited options, generally only bitcoin or personal checks, according to the researchers.