Malaysian authorities have identified the substance that killed Kim Jong-nam at Kuala Lumpur airport as VX, which is classified as a weapon of mass destruction. Bruce Bennett, a defence expert with the Rand Corporation, dissects how this could have happened.
VX is an extraordinarily potent chemical weapon. About 0.01g - less than a drop - on the skin can kill. The chemical goes through the skin and disrupts the nerve system.
It is an oil-like substance; it would normally not mix well with water, which raises questions about how it was applied on Mr Kim without killing those who carried out the mission. This is the first time VX has been used in such a way, so there are plenty of imponderables.
The CCTV footage and police statements do not lay out the full sequence of events. Two women appear to assault him, at least one with a cloth wiping his face.
Malaysian investigators said the two women coated their hands with the liquid toxin and wiped Kim's face afterwards. But if that were the case, they would have died immediately.
So if a liquid was sprayed or wiped on Kim Jong-nam it is likely that it did not contain VX and that would help to explain why the women seen accosting him did not die despite apparently getting the liquid on their hands.
In that case it appears likely that a very small quantity of VX - possibly just a drop - was actually on the cloths used by the women to wipe his face.
We do know now that one of the women involved has been vomiting since the attack.
The perpetrators would have really wanted to practise with this to make sure the drop touched Kim, and that they did not touch the drop. And that is what we are told happened.
Police say that they are believed to have repeated this move in shopping malls ahead of the actual event on 13 February.
As the drop absorbed into Kim Jong-nam's skin, it would have started affecting his nerve system, causing symptoms that take effect within minutes. The subject will experience coughing, chest tightness, blurry vision, fatigue and eventually seizures as the nervous system is shut down. He is likely to have died within minutes.
It appears that the North Korean government may have felt that they could claim the body and avoid an autopsy, thus denying the outside world knowledge of what had happened.
Malaysia has proven diligent in insisting on an autopsy and clearly North Korea failed in its efforts to prevent this. This has led to a row between the two nations, Malaysia being one of few that had diplomatic relations with the North.
So how could the VX have actually got into Malaysia?
Because the quantity required to kill is extraordinarily small, it could have been smuggled into Malaysia in a cartridge in a pen or some such thing. The security forces would have had no idea it was being smuggled in unless someone had tipped them off, which clearly did not happen.
It is unlikely that VX was made in Malaysia - it is not something that can be made safely in a kitchen sink.
Of course, we don't know for sure that North Korea made it and it is also possible they may have purchased it from a third country. There is both a US and a Soviet/Russian version of VX and it will be interesting to see which version was eventually used.
It would take sophisticated laboratory analysis to tell them apart, which may already have been done.
VX is extremely stable - like oil, it does not evaporate quickly. That made the VX safe on a cloth or some other surface until it touched human skin.
But this use of VX, unheard of previously, is a serious violation of international standards. The fact that it was used in a foreign country means that Malaysia and other countries will be both appalled and furious.
Of key importance will be how China responds. After all, China was reportedly providing protection for Kim Jong-nam.
If North Korea seriously violated international law, China should presumably do more than just cut off imports of North Korean coal.
China has the opportunity to punish North Korea and thereby hopefully deter it from carrying out this kind of attack again.
A broader Chinese trade embargo would hurt North Korea seriously.