Four men in the UK have been arrested over their alleged roles in the online marketplace Silk Road.

One of the suspects is in his 50s and from Devon while the others are in their early 20s and are from Manchester. They were detained by officers of the new National Crime Agency (NCA) shortly after the FBI arrested Ross Ulbricht - the suspected creator of Silk Road.

The NCA has warned that other people in the UK suspected of being "significant users" of Silk Road will be arrested over the coming weeks.

Keith Bristow, director general at the NCA, said: "These arrests send a clear message to criminals; the hidden Internet isn't hidden and your anonymous activity isn't anonymous. We know where you are, what you are doing and we will catch you."

He went on to say criminals can't completely erase their digital footprints, no matter how technology-savvy they are.

"They will always make mistakes and this brings law enforcement closer to them. These latest arrests are just the start; there are many more to come," Bristow added.

Andy Archibald, head of the NCA's National Cyber Crime Unit, said the NCA is committed to tackling the 'dark' or 'deep' web and those who exploit it. He added:

"These criminal areas of the Internet aren't just selling drugs; it's where fraud takes place, where the trafficking of people and goods is discussed, where child abuse images are exchanged and firearms are traded.

Stopping this element of serious and organised crime will go a long way to protecting the public."

It is not clear how much data the NCA possesses – for example whether it can see exactly which products each of the suspects purchased from the site.

While illegal drugs are reported to have made up 70% of the items sold on Silk Road, some other seemingly innocent items have also been up for sale on the site, including digital goods, jewellery and home and garden supplies. However, it seems unlikely the NCA will accept any claims from the suspects that their frequent visits to the site were merely to buy these innocuous goods.