Video report by ITV News Correspondent Paul Davies
Police in Malaysia confirmed the highly toxic chemical VX was used in the death of North Korean leader Kim Jong-un's half brother Kim Jong-nam last week.
But what is VX and what makes it so dangerous?
ITV News asked chemical expert Hamish de Bretton-Gordon, a former commander of the UK's Chemical, Biological, Radiological and Nuclear (CBRN) Regiment, to tell us more about this deadly chemical.
What is VX?
VX is "the most deadly chemical weapon ever produced", says Mr de Bretton-Gordon.
A highly toxic nerve agent, it was first developed in the UK in the 1950s but under international agreements it was "supposed to be all destroyed and outlawed".
How toxic is VX?
VX is so toxic that even a microscopic amount can kill.
"One touch would be enough to kill", says Mr de Bretton-Gordon.
How does VX take effect?
According to the expert, VX is primarily a "touch hazard" so you would need to be in physical contact with it for its full impact to become clear.
It can be dark in colour and highly viscose - in its unpure form- and has very high boiling and vapour points.
But what does it's toxicity mean in numbers?
An indicator of just how toxic VX is can be reflected in figures.
Mr de Bretton-Gordon says that taking 1 as an example toxicity figure for Chlorine, then Sarin would be 4,000, and VX a much higher 20,000.
How did VX come to be involved in Kim Jon-nam's death?
Mr de Bretton-Gordon told ITV News "obviously it's been smuggled in to Malaysia" and as it's the sort of chemical you "can't make in your garden shed" you'd need a professional laboratory which "points very clearly at North Korea" being the source.
He said: "We all suspected North Korea had a chemical weapons programme and this all confirms it.
"The prospect of one of their missiles being fired with VX on it is horrific."
So how come the women who allegedly contaminated Kim Jong-nam were not effected by VX?
Mr de Bretton-Gordon says that he thinks the women must have been wearing gloves.
He said: "One assumes they were wearing gloves"
But he can not explain why one woman is now reportedly sick and the other not.
"That is the question. Why didn't she die and why didn't anyone else die?", he said.