Who is poisoning Russian dissidents?

This story will probably become really compelling in a few days when it starts to make a little more sense. But right now it's just kind of annoying.

Why have "traces of the man-made isotope turned up in at least a dozen spots across this capital city and in several aircraft."

It sounds like the guy doing the poisoning took a stroll around London for a few days with the radioactive crap falling out of his pockets like loose change.

And whatever happened to Arsenic? Or just throwing someone off of a bridge?