Whoever contaminated ex-Russian spy Alexander Litvinenko with polonium-210 knew what they were doing and had to be connected to get the substance in the first place.

From the BBC story:

Mr Litvinenko's death on 23 November was linked to a "major dose" of radioactive polonium-210 found in his body.

Traces of radiation have since been found at five locations around London, including a sushi restaurant and hotel visited by the deceased.

But the radioactive substance implicated is as difficult to obtain as it can be to detect.

Polonium-210 occurs naturally in the environment and in people at low concentrations. But acquiring enough of it to kill would require individuals with expertise and powerful connections.

Professor Nick Priest, one of the few UK physicists to have worked with polonium-210, told BBC News that just one milligram (a thousandth of a gram) of the radioactive substance could have been responsible for Mr Litvinenko's death.

Higher doses than that would have killed the former KGB agent more quickly.