Brazil's federal police has announced that a nationwide investigation over the course of a year into the circulation of child pornography among users of private "darknet" networks has resulted in the arrest of 55 people — including teachers, priests, and at least one member of the military.

"This is the first time that the Brazilian police has done an operation like this, seeking targets in the darknet," Rafael França, coordinator of the aptly named Operation Darknet, told VICE News.

Darknet networks allow users to share files anonymously among a group of peers on the "deep web," which exists like a second internet that is invisible to most web users.

"In a nutshell, it's is a way of accessing or sharing info that isn't indexable," William Beer, an online security expert who leads the consulting firm Alvarez & Marsal's South American cyber-protection practice, told VICE News, referring to the process by which conventional search engines like Google or Bing "index" and retrieve information.

"When we think of the web, we think of interfaces like Google for finding information," he explained. "But for information on the dark web, that isn't possible — it's all hidden. The search engines and interfaces that the general public use don't apply."

Beer said that people typically use software like Tor, a special browser that hides a computer's IP address, to anonymously share information online.

"The anonymity of the darknet makes it much easier for criminals to access and share child pornography," França said, though he was quick to emphasize that not all darknet users are criminals. Journalists, bloggers, and political dissidents also employ such networks to communicate and bypass political censorship.

"It is a tool, used for both good and bad things," Thiago Tavares, president of Safernet, a Brazilian NGO working to combat crimes and human rights violations on the internet, told VICE News. Last year's bust of the Silk Road drew attention to the existence of darknet drug markets.

"These people arrested for child pornography were using it to commit crimes," Tavares added. "But there are many in Brazil and abroad using it to protect human rights. So we should never condemn the tool."

The police issued 100 arrest warrants throughout 18 states and the Federal District as part of the investigation, so more arrests are expected. They also relayed information about suspected child pornography traffickers to authorities in Mexico, Venezuela, Colombia, Italy, and Portugal.

França explained that the police had downloaded a special browser that hides the IP address of the user in order to access the darknet, though he wouldn't confirm that it was the Tor browser. In order to infiltrate the illicit file-sharing communities, he and six other officers went undercover as online pedophiles.

"We found and made contact within communities that are for people interested in child pornography, and gradually began communicating with them," he said. "Gaining their trust was a very difficult job, they were very suspicious. Every day they were talking about the police, saying, 'Are there cops among us?' "

A psychologist was on hand to support the officers, who in the course of the investigation had to view the disturbing images and videos that the users were sharing.

"This was very distressing," he said.

Using "unprecedented research methodology and developed tools," França said his team was eventually able to crack the anonymity of dozens of darknet users sharing child pornography. He noted that he wasn't at liberty to discuss the process in detail.

"This is our secret," he said. "I cannot talk about it."

The federal police has so far rescued six children from situations of sexual abuse and rape, including a baby whose father had bragged online that he would abuse her daughter as soon as she was born.