After a man was arrested in June for allegedly operating what appeared to be Japan’s largest child pornography website, experts believe police are making progress in unmasking anonymous predators who engage in illegal and immoral activities on the darknet.

Facing a huge challenge because of the difficulty of identifying people who conceal their location by lurking in the recesses of the internet, the Kyoto Prefectural Police secured their breakthrough due to flaws in the exploitative website operator’s own procedures.

Police were able to track down Mitsugu Ono, a 59-year-old man who they suspect operated the site named Lollitter2.onion, through data analysis and by contacting users who, like the investigators, had gone through the site’s compulsory registration process.

Kyodo News reporters used the software program Tor to gain an understanding of the murky parts of the internet where sites hosting child porn, illegal markets and other illegal and legal services that have users who seek anonymity. One Japanese child porn site’s home page that reporters accessed showed young foreign girls, most appearing to be under elementary school age, clad in underwear and bathing suits. There were forums, rankings, and even banner ads for Japanese anime.

Tor, which stands for The Onion Router, directs internet traffic through a network structured like the layers of an onion. It encrypts data, including IP addresses, multiple times and routes it on a random journey across multiple systems, making it nearly impossible to track and, therefore, identify the parties involved.

Although mainly thought of as a hotbed for criminal activity, the darknet is also used by whistleblowers in countries where free speech is not guaranteed and censorship makes sharing anti-government views a potentially dangerous activity.

According to the Kyoto police, a 2015 study conducted by a British university found that around 45,000 hidden services could be accessed using Tor, including about 900 child porn sites.

“It is just the tip of the iceberg,” said one high-ranking police official. “No one really knows what the real figures are.”

The darknet — which is also used by criminals to trade firearms, narcotics, counterfeit money and other contraband — has become a refuge for pedophiles as authorities worldwide have ramped up efforts to eradicate child pornography from the open internet.

The same university study also concluded that more than 80 percent of darknet traffic is to sites showing underage pornographic material. According to police investigators, the man arrested in the case operated more than 40 child porn websites, the most such sites run by an individual that Japanese authorities have ever discovered. On them he had posted some 210,000 images of children.

Police initially struggled to make progress in the case due to the inherent difficulty of investigating on the darknet. But they were able to identify the man through data analysis and interviews with members of the site.

From the membership list, police were able to identify approximately 50 more Japanese users and are set to refer them to prosecutors on suspicion of violating the law prohibiting child prostitution and child pornography.

Last year, Japan reported a record number of child pornography cases, reflecting a police crackdown that was enabled by obtaining through a separate investigation a massive list of child porn purchasers, including teachers and politicians.

The Tokyo Metropolitan Police arrested four people in May 2017 over suspected sales of child porn DVDs on a membership website, leading to a series of further arrests. Tetsutaro Uehara, a professor who specializes in cybersecurity at Kyoto’s Ritsumeikan University, suggested the June arrest of Ono is only scratching the surface.

“(Police) haven’t actually broken through the Tor system, but this was a major achievement in poking a crack in it. This offers an opportunity to actually reveal how the distribution of child porn happens, which can become a deterrent for future sex crimes,” he said.