Polonium-210--Another Possibility

Maybe it wasn't Putin's work:

A J Strata has been following all the public information as it has been forthcoming these last nine weeks, and is firmly convinced that Alexandre Litvinenko died as part of an elaborate and well financed polonium smuggling operation. It seems much more likely that rather than an insanely expensive assassination, one of the transport personnel simply falls victim to some combination of complacency, curiosity and clumsiness on the third delivery.

There really are only two likely reasons to smuggle in quantities of a volatile and deadly rare synthetic molecule. The first is to restore the decayed trigger on an older nuclear weapon and the second (and perhaps a fallback option) is to use the material as a dirty bomb in the heart of a western city. As A. J. Strata points out, the authorities have some idea of how much was leaked in the accidental contamination, but there really is no good understanding of how much is safely in the wrong hands.

Perhaps spread around with a release of the natural-gas "smellificator" ingredient?

HT: Random10