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A child porn site seized by the FBI was allowed to run for a time so agents could track usersFlickr/Caroline

It has emerged that FBI agents who hacked into and seized one of the biggest child pornography web sites used the controversial tactic of continuing to run the site for a short time, allowing criminals to download illicit photos and videos of children.

The newspaper USA Today reported how officers used cutting-edge hacking methods to hunt the customers and owners of a site on the dark web, a largely secret world of the Internet that can't be accessed by conventional means such as a Google search.

After agents seized the computer server running the bulletin board site, Playpen, from a web host in North Carolina, the FBI continued to run the operation from its own servers in Virginia for 13 days from 20 February to 2 March, 2015, the Justice Department acknowledged in court documents.

Playpen had some 215,000 users and links to 23,000 child porn images and videos, some of which contained extreme abuse. The site also included close to 9,000 files that could be downloaded directly, as well as instructions on how to avoid detection when accessing child pornography.

Once the site was seized by the FBI, a Flash application was secretly installed on the computer of any user who accessed the site that sent data about that client straight to the FBI. This bypassed the Tor anonymity network which usually successfully hides users' identities, according to Motherboard Vice. Close to 100,000 registered users visited the site while it was in the control of the FBI.

The FBI strategy of keeping the site online to catch clients, marked a radical departure for the bureau, which had generally blocked any such images from being downloaded, given that every such image harms children. Once a photo or video is downloaded, moreover, law enforcement loses future control of its use.

But FBI officials believed the risks were worth catching the criminals. "We had a window of opportunity to get into one of the darkest places on Earth, and not a lot of other options except to not do it," Ron Hosko, a former senior FBI official who was involved in planning one of the agency's first efforts to take over a child porn site, told USA Today. "There was no other way we could identify as many players."

"The government always considers seizing an illegal child pornography site and removing it from existence immediately and permanently," said Justice Department spokesman Peter Carr. "While doing so would end the trafficking of child pornography on that one website, it would do nothing to prevent those same users from disseminating child pornography through other means."

Still, he said: "The decision whether to simply shut down a website or to allow it to continue operating for a brief period for a law enforcement purpose is a difficult one."

On two other occasions, in 2012 and 2013, the agency allowed such sites to run for brief periods but the sites had far less traffic. So far 137 Playpen users have been charged with crimes.

"At some point, the government investigation becomes indistinguishable from the crime, and we should ask whether that's OK," Elizabeth Joh, a University of California Davis law professor who has studied undercover investigations, told USA Today.

However lawyers have warned that the tactic presents an opportunity for a defendant to claim entrapment. "What the government did in this case is comparable to flooding a neighbourhood with heroin in the hope of snatching an assortment of low-level drug users," one defence lawyer recently told a judge in a case that has yet to be decided.