The "dark web" services used by criminals will continue to evolve in an attempt to evade authorities, the UK's cybercrime boss has warned.
Last week, notorious drugs market place the Silk Road was shut down after a lengthy investigation.
Andy Archibald, interim head of the National Cyber Crime Unit (NCCU), said officers identified individuals who were using the site.
But he said new methods were needed to keep up with the threat.
"[Online anonymity service] Tor evolves, and will resecure itself," Mr Archibald told the BBC's technology correspondent Rory Cellan-Jones.
"The success we've had may not necessarily mean that by the same routes and same approaches we can get into other criminal forums.
"We have to continually probe and identify those forums and then seek to infiltrate them and use other tools.
"It's not simply a case of because we were able to infiltrate Tor on this occasion that we'll be able to do it next time around as well."
Mr Archibald's comments came as the NCCU announced its first conviction. Twenty-seven-year-old Olukunle Babatunde received a five years and six month prison sentence.
The man, from Croydon, south London, pleaded guilty to using "phishing" scams in an attempt to defraud banks, financial institutions and their customers.