Why crimes on Skype leave witnesses but no evidence

The case was the most graphic illustration of a recent phenomenon - horrific acts committed on what is known in the trade as voice over internet protocol.

Martin Geddes, a Skype expert, says: "There are more minutes of Skype usage per month than the total of international phone calls.

It is far and away the biggest and best-known of the VoIP (Voice over Internet Protocol) providers, nevertheless there are others - Ooma, Viber, Viatalk, for instance, and any number in China.

Mr Geddes explains: "Skype is just a piece of software and such as you don't expect the internet to remember every computer game you have played so you can't expect it to record your conversations.

Users can record Skype calls, however for this they need to buy add-on software. This tends to provide the ability to record up to 15 minutes of conversation at a time.

"People use this sort of software to record for business purposes or to record family calls for people who can't be there themselves," says Phil Wolff, editor of the Skype Journal blog.

In 2009 it was reported that organised criminals, arms and drugs traffickers, and prostitution rings in Milan, Italy, were using Skype in order to frustrate investigators who regularly tapped their phones.

Mr Geddes says the encryption of Skype has brought about a "fundamental change in the balance of power between governments and people" because they cannot tap or bug VoIP calls.