>> reporter: why if he had died 12 hours earlier, would it have made any difference? >> because they wouldn't have found out, they would have marked the death 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. >> it's an almost perfect murder weapon. polonium has no smell, little taste, and without specialized equipment. it's undetectable. >> if you're not looking particularly for polonium, then you would not able to discover it. do you know? it will be everywhere, but you don't know this. >> reporter: the amount that killed litvinenko was slipped into something he ate or drank and was no larger than a grain of salt, but that's still a thousand times the lethal dose. and that tiny bit of polonium