The BBC report that UK and US intelligence agents foiled a chemical bomb plot in the UK. Apparently the plot was involves detonating a combination of explosive and osmium tetraoxide, [OsO4].
The target is thought to be in crowded areas, possibly within a confined area, perhaps in London. The plotters apparently did not obtain any of the tetroxide, perhaps because it costs around UK pounds 100 per gram!
Not totally clear why osmium tetroxide was chosen. Certainly it is unpleasant, corrosive, toxic, irritant, volatile, and a stain. It reacts with alkenes - hence the staining properties, and must be handled properly in the lab.