As Malaysian toxicologists reveal that the banned nerve agent VX was used in the airport assassination of Kim Jong-Nam, here are some key questions and answers about the deadly weapon of mass destruction.
What is it?
Code-named by the US scientists who mass produced it, VX is an organophosphate compound and one of the deadliest chemical agents ever manufactured.
Stockpiled by the US in huge quantities during the Cold War, VX is perhaps 10 times as powerful as the Sarin toxin.
Odorless and clear when pure, it has the appearance of motor oil and is stable enough to be transported. It is also hard to detect, an advantage for a would-be assassin.
Downsides are that it lingers, potentially contaminating areas for long periods of time.
"It can kill an adult weighing 70 kilogrammes with just five milligrammes on the skin," said Yosuke Yamasato, former principal of the Japan Ground Self-Defense Force Chemical School.
"It's unbelievable that the executors of the crime used it with their bare hands - they must have not known the material was VX."
What does it do?
It strikes the nervous system fast. A high dose can kill in minutes when inhaled, as the blood vessels in the lungs rapidly spread the compound into the bloodstream and vital organs.
Nerve agents over-stimulate glands and muscles, leading them to quickly fatigue and become unable to sustain breathing.
Symptoms depend on dosage and whether it is inhaled or introduced through the skin - the slower form of poisoning.
Exposure to low doses is survivable.
But more serious contamination is fast-acting and often gruesome. People exposed to the toxin may become short of breath and nauseous in minutes, or at a higher dose experience seizures, heart failure and a total shut down of the respiratory system.
There are antidotes but treatment must be immediate. US soldiers carried kits to inject themselves with antidote during the first Iraq War.
Where does it come from?
The compound was first created in a British laboratory in the early 1950s. But American scientists honed its potency during the Cold War arms race with the Soviet Union.
Tens of thousands of tonnes of VX were churned out at Newport Chemical Depot in Indiana -- a stockpile that was finally destroyed in the late 1980s as the Cold War ended.
Accidental leaks have been reported in the US and Japan. It has been deployed as a war weapon infrequently but with devastating effect.
Residues found on site suggest Iraqi dictator Saddam Hussein may have used VX among a cocktail of chemical weapons he rained down on the Kurdish town of Halabja in 1988 killing at least 5,000 people.
In 1994 VX was used by Japan's Aum cult to murder an office worker in Osaka, and in the attempted murder of two other people.
VX is listed a weapon of mass destruction by the United Nations.
Under the international Chemical Weapons Convention 1997, countries are allowed limited stockpiles for research purposes only but must declare them and are obliged to progressively destroy their supplies.
"North Korea is not a signatory to CWC, so it's no surprise if it possesses VX," Satoshi Numazawa, professor of toxicology at Showa University, told AFP.