Slate has a detailed examination of the Litvinenko murder, and the nature of polonium, the ultimate smoking gun.

Fairly damning evidence against Andrei Lugovoy:

What can be said at this point is that Lugovoy was shedding radioactivity before Sasha was exposed on November 1. For example, Lugovoy contaminated the leather sofa in Boris’s study when he visited him on October 31.

Lugovoy might not have put the polonium in the teapot himself, but he did carry it around.

It’s well worth a read.