Police face questions after "deep dark web" raids

Last week, a joint operation by the 16-member states of Europol, the FBI, US Homeland Security and US Immigration and Customs Enforcement (ICE) resulted in a shutdown of 27 "dark" websites and the arrests of 17 people allegedly in charge of those websites.

The sites operate on the Tor network (The Onion Router), a service that offers online anonymity by routing a person's journey on the web through various different computers and provides access to the "dark" or "deep" web, so-called because the sites don't show up in the results of conventional search engines.

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Among those sites was Silk Road 2.0, a site selling narcotics, made after the shutdown of the original Silk Road last year.

However, Tor is also viewed as an important tool for those involved in important causes such as activism in repressive regimes, which is one of the reasons why the network is funded by the US government.

After the closure many experts, including Tor developers, were left asking how it happened and what techniques were used to "de-cloak" people using Tor.

"We were as surprised as most of you," said a statement posted on the Tor Project blog. "We are left asking 'How did they locate the hidden services?' We don't know.

"In liberal democracies, we should expect that when the time comes to prosecute some of the 17 people who have been arrested, the police would have to explain to the judge how the suspects came to be suspects.

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"As a side benefit of the operation of justice, Tor could learn if there are security flaws in hidden services or other critical internet-facing services."

The Tor Project detailed a few possible scenarios of how the sites could have been compromised, including poor operational security and the "deanonymisation" of Bitcoin (tracing the movement of the digital currency).