Somewhere here on FR is an article (sorry, did not book mark) that figures he was smuggling the stuff to activate one of those suitcase nukes that needs refreshing.
Article from a published source? A few of pajama-wearing types have been posting that analysis here intermittently: polonium 210-beryllium 9 neutron sources.
I keep wondering whether a whole trigger was being smuggled, in which case there might should be beryllium contamination at the sites, too. (Might or might not--the two have to be kept separate until the ignition sequence for the plutonium bomb--if only the polonium containing part was breached, there might not be beryllium residue. OTOH, beryllium residue at the contaminated sites would kill the assassination theory, and be *very* scary, as there is no proof only one trigger was involved in the smuggling operation.)
No suitcase nuke would be using an alfa source with 138 days half-life [it would need continuous maintenance and replacement], while longer lived ones are available. More, the nuke source would not be in a form of aqueous solution, but as a well-encapsulated solid.