certificate as death unknown. he would have been put in the ground, and it would have been just a mystery. unknown -- unknown assailants. turn the page, move on. >> it's the key of this murder. polonium 210 was discovered and now we know exactly sasha was killed by polonium 210. >> reporter: it's an almost perfect murder weapon. polonium has no smell, little taste, and without specialized equipment, it's undetectable. >> reporter: the amount that killed litvinenko, slipped into something he ate or drank, was no larger than a grain of salt. but that's still a thousand times the lethal dose. and that tiny bit of polonium would have been enormously expensive. >> $8 million to $12 million to be able to get the portion that went into him. >> reporter: but who could get hold of such an expensive and exotic weapon? and just how did they deliver the fatal dose? when detectives went step by