Malaysian authorities said Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, was killed with the banned nerve agent VX at Kuala Lumpur airport on February 13. Preliminary toxicology tests detected the substance in swabs from his eyes and face. Kim sought help after a woman approached him and wiped something on his face, but suffered convulsions and died on the way to hospital. He had been travelling under the name Kim Chol and was reportedly returning to Macau, where he lived with his family.

Malaysian investigators accused two women, one Vietnamese and one Indonesian, of applying chemicals to Kim’s face while North Korean agents looked on. Police said one woman later suffered vomiting and argued that her immediate handwashing showed awareness that she had handled a toxin. Relatives and acquaintances maintained that the women had been deceived. Toxicologists questioned how the suspected attackers could have handled VX and survived, though one expert suggested an antidote was possible. Authorities were seeking several North Koreans, including an embassy official, and investigating how the substance entered Malaysia.

VX is a highly toxic, persistent, oily nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. A minute quantity can be fatal, and symptoms can include vomiting, seizures and paralysis. Its use raised public safety concerns because the airport had gone 11 days without decontamination; authorities subsequently said cleaning was underway. No passengers, airport workers or medical staff were reported to have fallen ill.

The killing triggered a diplomatic dispute. North Korea denied responsibility, challenged Malaysia’s investigation and blamed Malaysia for the death. It did not acknowledge that the victim was Kim Jong-nam, and a statement attributed to a North Korean legal body noted that the country was a nuclear weapons state. Malaysia warned the North Korean envoy that he could be expelled over his accusations.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, had once been considered a possible successor but had lived abroad for years and publicly criticized North Korea’s policies and hereditary succession. North Korea has not signed the Chemical Weapons Convention, which bans VX. Outside estimates put its chemical weapons stockpile at 2,500 to 5,000 tons and suggest it can produce thousands of tons annually. VX has also appeared in previous attacks: Japan’s Aum Shinrikyo cult used homemade VX against three people in 1994 and 1995, killing one.