A radioactive probe?

It's recently been reported that up to 30,000 passengers were being tracked for having flown on three London-Moscow route British Airways planes which may have been involved in the polonium-210 poisoning of Litvinenko. This raises the question of why a radioisotope was used for the attack when a more conventional poison would have sufficed.

Perhaps the attack was also a probe of Western capability to track and/or stop a radiological attack. It's clear that British Airways was not able to intercept the polonium en route.