Perhaps it was the Russian Mafia? They'd have access to the richest source of weapons-grade plutonium--in their own country, from military bases that are insecure, under-guarded and leaky. I'm sure the US Mafia would trade in fissionable materials if they had the chance--they'd sell their grandmothers' kidneys if they had the chance--but it's unlikely they'd have access to plutonium or uranium.