For decades, inside an unremarkable warehouse outside Los Angeles, one man gathered among the most remarkable collections in America: hundreds of thousands of letters, all of them worrisome. Some promised terrible violence. Others merely hinted at calamity. Some even included off-putting gifts like syringes of bodily fluid, animal corpses, and hair.

They were all carefully saved and catalogued by Mr Gavin de Becker, arguably the world's foremost expert on threatening messages.