When not detecting intelligence threats to oil rigs and dams, Sergio Caltagirone spends his spare time hunting a different kind of predator – traffickers trading in human beings, from war-torn Syria to sleepy United States suburbs.

The Seattle-based computer scientist, who previously worked for the US Department of Defense, Microsoft, and NASA, is one of a new breed of digital hacker sleuths who are saving lives by tracking down traffickers and rescuing victims on the internet.

"It's just like any other business in the world," said Mr. Caltagirone, who set up the Global Emancipation Network (GEN) with his wife, Sherrie, two years ago, to analyze data to help law enforcers counter human trafficking.

"If you know how to find it, you will see it almost everywhere – almost every major site has some component of trafficking in it," said Caltagirone, whose day job is with the industrial cybersecurity firm Dragos.

Opinion is divided over the rise of hacker sleuths who deploy their cutting-edge knowledge, skills, and experience to support governments that often lack the time, motivation, and innovative tools to tap into criminal slavery networks.

Human trafficking is among the world's largest international crime industries, with about 25 million people trapped in forced labor generating illicit profits of $150 billion a year – and one which is moving increasingly online.

The US-based National Center for Missing and Exploited Children said in 2017 that almost three-quarters of suspected child trafficking reports it received from the public involved the sex advertising website Backpage.com.

Backpage – described by campaigners as the country's largest online marketplace for child sex trafficking – was shut down in April and its founders were charged in a 93-count indictment, including knowingly facilitating prostitution.

But the years of lobbying that preceded the crackdown showed how authorities with limited digital expertise struggle to stop criminals who use technology at every stage of their business, from recruiting via social media to tracking victims via webcam.

"You have to know exactly where to go," said Sharon Nimirovski, head of White Hat, an Israeli cyber security firm staffed by former military intelligence agents.

"You have to go undercover and live the hacker cyber scene, know its structure, and pretend to be someone you are not in order to retrieve the data that you are looking for."

While the precise methods used by hacker activists are veiled in secrecy, Mr. Nimirovski said his team has used false digital identities to infiltrate hidden cybercrime sites to gather information on pedophiles.

"Just like the police work in the physical world, White Hackers act in the digital dimension," he said, adding that his White Hat Hackers – or hackers working for good – share the criminal evidence they unearth with authorities.

Dark web

GEN, which is run by volunteers, collects text and images from the open and dark web – a part of the internet invisible to search engines and only reachable using specialized software – to look for patterns that could indicate trafficking.

It shares this suspicious online activity for free, via its Minerva platform, with law enforcement, researchers, and anti-trafficking charities that often do not have the capabilities to trawl the online black market and message boards.

The software allows investigators to search through data from millions of – often hidden – internet pages using keywords, usernames, and phone numbers to find out what other sites their suspects visit and who else they communicate with, GEN said.

Digital evidence gleaned from visa blacklists, bitcoin transactions, and sex ads can help to bust traffickers by predicting where victims might go, via which routes, and who is likely to buy or sell them, experts say.

"The earlier you move into the kill chain, the more effective your disruption becomes, and the more people you ultimately save," said Caltagirone, GEN's technical director.

One of the routes GEN is tracking closely is that of people moving to Eastern Europe from Syrian refugee camps, often in the hope of finding lucrative jobs advertised on fake websites.

"Of course these victims are going to be very willing," said Caltagirone, highlighting how technology has not only made it easier for migrants to reach Europe, but also enabled criminals to trick people into trafficking themselves and their families.

"This is where you'll get parents who sold their children."

Online vigilantism

Yet caution is required as hackers may not have the training needed to collect evidence that is admissible in court, said Nazir Afzal, a lawyer and former British chief prosecutor who fought major cases involving sexual slavery and child abuse.

"If, in some [human trafficking] cases, hacking leads to the early detection of a big vulnerability – that's fine, I suppose," said Rob Wainwright, a cyber security expert and ex-head of Europe's policing agency Europol.

"But we have to be very careful about encouraging online vigilantism," he added. "We have to do things in the right way."

But others say that private digital sleuths can play a vital role, particularly when working together with the police.

"Law enforcement, in many countries, either lack the financial resources or human resources or both needed to perform cybercrime investigations efficiently and swiftly," said Joyce Hakmeh, a cybercrime expert with the think tank Chatham House.

"Most, if not all, cybercrime investigations require public-private partnerships and getting the right experts on board," she said in emailed comments, adding that ethical hackers working with the police can have a big impact in cracking cases.

GEN is confident that cyber hackers have a key role to play in combating trafficking – and boosting prosecutions, which numbered about 9,000 in 2016, according to the US government.

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"We're not here to save the world," said Caltagirone. "But GEN is here to make people who are saving the world even better at doing it."

This story was reported by the Thomson Reuters Foundation.