Dark-net sites dealt a significant blow' says Europol head

The closure of 400 sites believed to be selling drugs and weapons online is a "significant blow" to dark-net operators, according to the head of Europol's cybercrime unit.

The sites, including Silk Road 2.0, operated on the Tor network - a part of the internet unreachable via traditional search engines.

Troels Oerting from Europol told the BBC's Anna Holligan how the joint police operation between 16 European countries unfolded.