Cases of lone killers embarking on slaughter sprees

Have occurred with frightening regularity since the late 1980s. People like Michael Ryan, Thomas Hamilton, Martin Bryant and Oklahoma bomber Timothy McVeigh. What drives these men - and it is always men - to turn on friends, family and strangers in acts of senseless rage and slaughter?

In the wake of the summer of 1999, in which four incidents of spree killing shocked the world, this is a look at a chilling new trend of brutal and indiscriminate killing that blights our "civilized" society.