What are nerve agents and how do they work?
Nerve agents have been used to deadly effect in assassinations and conflicts of the past.
The toxic substance disrupts signals in the nerves, causing debilitating side effects which can be fatal.
OTHER CRIME NEWS: Man dies after being stabbed in PrestonVladimir Putin - now in the frame following the attempted murder plot in Salisbury - drew international condemnation for defending the Syrian regime after it launched a suspected Sarin gas attack on its own people last year.
Earlier this week the United States said North Korea used the toxin VX to assassinate leader Kim Jong Un's half-brother in Malaysia in 2017.
Here, the Press Association answers questions about the lethal substances.
- What are nerve agents?
Nerve agents take different forms. Contrary to popular belief, they are a liquid, rather than a gas, and can seep through the skin.
They were first discovered by accident in the 1930s, when scientists were attempting to find a more cost-effective pesticide.
It proved to be incredibly toxic and posed a risk to mammals, leading it into the hands of the German military, which crafted a weapon of war.
Russia came across such chemical agents for the first time when they swept into East Germany following World War Two and took control of the plants where they were made.
- How do they work?
The toxins interfere with the central nervous system, causing the body to become overstimulated.
Different forms of it have evolved, including Sarin, VX and Tabun, all of which have very similar structures and appear to work in the same way.
Dr Simon Cotton from the University of Birmingham said: "They interfere with the transmitting of nerve impulses.
"Our bodies use a molecule called acetylcholine that migrates the gaps between cells - it goes from one cell and slots into the second and triggers a nerve impulse.
"The body has to get rid of acetylcholine that is docked in the receptor because it builds up and you keep getting nerve impulses and become overstimulated.
"Our bodies have got an enzyme that breaks up acetylcholine called acetylcholinesterase - what a nerve agent does is bind to the acetylcholinesterase and stops it from working."
- What are the symptoms?
If you have ever sprayed insect repellent at a fly, you might have seen it drop to the ground and lie on its back, legs twitching.
This is the result of nerve agents taking hold.
Dr Cotton added: "It is basically overstimulated to death because of the build-up of acetylcholine."
Twitching, spasms, heart failure and respiratory arrest are among the more common side effects.
- How much is needed to make an impact and how long does it take?
Part of the allure a nerve agent holds for assassins is that only tiny amounts are required for it to take effect.
It is so toxic that it would usually be transported in something tightly sealed and those who apply it will need protective clothing.
"A drop would be needed, that is all, it is that sort of scale - a tiny scale," Dr Cotton said.
Victims will not have to wait long until they realise something is wrong, as was demonstrated in a Sarin attack on the Tokyo subway which left 12 dead in 1995.
"People would start feeling funny within a few seconds, but it would depend on the dose," Dr Cotton added.
- How is it applied?
Again, this can vary. Doses might be turned into an aerosol can spray, for example.
In the Tokyo attack, liquid sarin was placed in plastic bags which were pierced by umbrellas with sharpened tips.
When Kim Jong Nam was killed at Kuala Lumpur airport last year, a cloth doused with VX was smeared on his face.
- Can victims be treated?
Antidotes do exist, including a medication called atropine - which works by blocking the receptor that acetylcholine usually binds to.
- How easy is it to procure?
In the UK, the ingredients required to create a nerve agent are carefully regulated.
Scientists can access some of the components for perfectly legitimate purposes, but will have to explain what their intentions are with it.
Overseas regulation can be less stringent.
Dr Cotton said: "Sarin and VX aren't that difficult to make if you have access to the chemicals you need."