Malaysia has said the substance used to kill Kim Jong-nam, the half-brother of North Korea's leader, was VX, a hugely potent nerve agent which is banned internationally.
What is VX?
The clear, amber-coloured, oily liquid is tasteless and odourless, making it hard to detect. It is the most toxic of all known chemical warfare agents - about 100 times more potent than sarin.
A drop of VX on your skin can kill a person within minutes.
VX is originally an American code name for the official chemical name: S-2 Diisoprophylaminoethyl methylphosphonothiolate. It is banned under the 1993 Chemical Weapons Convention (CWC), meaning states should not manufacture it and should destroy their stockpiles.
How does it work?
VX can be absorbed into the body by inhalation, ingestion, skin contact, or contact with the eyes. It works by penetrating the skin and disrupting the transmission of nerve impulses.
Its only use is as a chemical weapon. The nerve agent can be disseminated in a spray or vapour, or used to contaminate water, food, and agricultural products.
What are the effects of exposure?
The symptoms are similar to pesticide poisoning, and start immediately after exposure.
A person will experience coughing, chest tightness, wheezing, or shortness of breath. As their vision becomes blurry, fatigue or headache will follow.
Other symptoms include runny eyes and nose, abdominal pain or diarrhoea, nausea and vomiting, abdominal pain, muscle twitching or a seizure.
If the dose is high enough it kills by shutting down the nervous system, causing seizures.
Clothing can carry VX for about 30 minutes after contact with the vapour, which can expose other people.
Has VX been used before?
Only Russia and the US have admitted owning VX stockpiles, but other countries are believed to have it.
Cindy Vestergaard, chemical weapons expert and senior associate at the Stimson Center, says Iraq "definitely produced VX" during the 1980s, but there is no evidence confirming its use.
In 1969, 23 soldiers and one civilian were sent to hospital after the VX nerve agent stored in a US military base in Okinawa, Japan, leaked.
The case created "huge outrage in Japan", Ms Vestergaard told the BBC, causing the US to "publicly state that they had chemical stores abroad".
She said if Kim case is confirmed, it would be the first ever instance of VX being used in an assassination.
Does North Korea produce VX?
North Korea - along with Egypt and South Sudan - has neither signed nor ratified the CWC.
Defence analyst Lee Il-Woo at the private Korea Defence Network told AFP it "is believed to have a large stockpile of VX, which can easily be manufactured at low cost".
South Korea's defence ministry said in a 2014 Defence White Paper that the North had began producing chemical weapons in the 1980s, and was estimated to have about 2,500 to 5,000 tonnes in stock.
North Korea has chemical weapons production facilities in eight locations including the north-eastern port of Chongjin and the north-western city of Sinuiju, it said in the 2012 edition of the document.