Professor Paul Mullen

They also share, many of them, a fascination with guns.

So they are caught up in fantasies of glorious battles and glorious death.

And this long predates the point at which they start thinking about the hopelessness of life and how badly they've been treated.

So those things come together.

Well, I mean, there are a small group in this lot who are actually mentally ill.

They have an illness of a schizophrenic type, about 10%.

But that's no higher proportion than among homicide offenders anyway.

So they're a little bit more difficult.

They have the same features, but they're driven by mental illness.

But if we put the 10% to one side, how do we do something about the rest?

The first thing is these people almost always put their hand up well in advance.

You can tell from their searches on the internet, their obsessive search about previous mass killers.

They also often put out statements on the internet.

They leak, as it were, information.

They don't talk to many people.

but sometimes they'll put it into a school essay or a college essay or they'll get angry with someone and they'll say what they're going to do.

So they actually, you get hints of what they're planning.

So it is possible to pick up by monitoring and responding to information.

Now, of course, you've got to identify vast numbers of people, well, not vast, large numbers of people who are not going to go down this route.

So what do you do?