And because there is so much of this rapidly evolving and changing digital evidence it produces a range of challenges for investigators.

We have police services trying to keep up with their capacity to access and process and understand what the digital evidence is saying.

And we also have a capacity to process, like the physical capacity to process this data.

And then on top of that, we have a workforce who's not necessarily trained to understand what this evidence means or what these digital traces mean in terms of their case, in terms of integrating it into an investigation in terms of the technical language that comes with analysing such data, in terms of understanding the implications of a particular piece of digital evidence.

For example, what does it mean that a photo has come from WhatsApp rather than come from the phone itself? Because those have different implications for progressing an investigation and for prosecuting a case.

So we have training issues, we have integration issues and we have capacity issues already.

There's another significant challenge presented by digital evidence and it's one of geography.

So as you know, the internet's not located in a particular geographical place.

They'd been fast friends for more than 20 years.

She was the one person that I was never afraid to share my secrets with, and she was the same with me.

We'd laugh together, we'd cry together.

Unconditional love and support.

And so during those months when Carrie was missing, Amy tried everything to get Carrie to talk to her.

I'd send messages to Carrie's cellphone.

I'd send her Facebook messages.

And most of the time I didn't get a response.