A kidnapper wishing to make an anonymous post on a website would have a range of methods at their disposal to hide their tracks - providing they had a sufficient amount of technical expertise, experts said today.

They were reacting to the chilling news that British security services had foiled a suspected plot to kidnap a British Muslim soldier and post a video on the internet of his beheading. Details of the alleged plot are still sketchy and nobody has been charged.

As a general rule, it’s possible to find out the identity and location of a computer which has made an offending post. This is because every device connected to the internet has a unique identity, known as an IP address, and the name and address of the user will be known to the ISP from when the user registered.

In the case of a YouTube video, for instance, although videos are ultimately hosted on the company’s own server, they will orginally have been uploaded from a home computer, whose IP address would be known to YouTube. In turn the ISP which administers the user’s account should be able to reveal their details, it compelled to do so.

There are, however, exceptions.

People can connect to the internet via a ‘proxy server’, in which case they would be able to make a post using an IP address which would not disclose their identity, making it difficult for police to track.

Awareness and use of proxy servers was "still pretty low," according to one expert, but someone with "a reasonable degree of computer expertise" would be able to organise for their connection to the internet to go through a series of servers, making identification of the end computer tricky.

The Serious Organised Crime Agency, Britain's newly-created answer to the FBI, would not comment on whether the identity of criminals using proxy servers could be traced.

A sophisticated hacker could also use a network known as a ‘botnet’ to hijack an existing site and host the material there, while the victim remains oblivious to the intrusion.

"An underground organisation called 'wikileaks' is also developing a protocol which would enable employees in oppressive regimes to post incriminating documents online without fear of being traced."

A separate question is whether the site on which a post has been made can be traced and compelled to disclose details.

Sites hosted in Britain are subject to British law, but if a site is hosted on an ISP in another country, the ability of police to compel the ISP to reveal details of the site’s users will depend on the laws of that country.

Police will generally be able to establish the identity of the hosting ISP via the organisation which has registered the site, known as an internet registry, which is separate to the ISP.

Sites which have ‘.co.uk’ domain names, for instance, must be registered with Nominet, a British company, which would usually be able to refer police to the server which hosts the site.

A spokesperson for the Met’s specialist crime unit said: "Websites can always be tracked.

When they are set up, various information is written into them which enables their location to be established."

A spokesperson for GetSafeOnline, the campaign group, was more circumspect: "A naive user could probably be traced but a sophisticated one can do a lot to cover their tracks and an ISP has to have a considerable amount of resources and motivation in order to pursue them."

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