According to the The Spy Who Billed Me, the killers of Mr. Litvinenko probably came from the Russian FSB, which will be a shoo-in for the Inspector Clousseau Award based on their execution of this assignment. You can read the details in Why You Should Never Use Your Shoe-Phone After Stepping In Polonium-210.

The bungling spies apparently dropped the polonium on the floor of a hotel room, and then turned on the light leaving traces on the wall switch. Their radiation trail was easy to follow across London according to The Telegraph.

the murderers may not have known how clearly their weapon of choice would show up, like a glowing trail of footprints around London

A reader at Firedoglake describes how difficult Polonium is to work with.

Polonium is the most aggressive moving radioactive isotope. To get Livitnikov to inhale this stuff would be very easy, just get some anywhere on his body and he would be internally contaminated. When I used to work with this isotope, it would move against air currents, it could not be contained, in fact, we had to have a ridiculous number of safety procedures in place to deal with the threat of loss of containment and even then we worried. We worked with this stuff only in glove boxes, and then in full protective gear besides. 100mgrams placed on his lapel would contaminate everything he touched and anyone he saw. A point to remember is that the button man is probably also very very ill, or dead by now too…