Malaysian authorities say 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 tests detected the substance in swabs from his eyes and face. Kim, who was travelling under the name Kim Chol and lived with his family in Macau, sought help after two women approached him and wiped chemicals on his face. He suffered convulsions and died on the way to hospital.

Malaysia says North Korean agents orchestrated the attack. Police detained the two women, one Vietnamese and one Indonesian, and sought several North Koreans, including an embassy official and an employee of the state airline. One of the women later vomited, according to the police chief. Relatives and acquaintances have argued that the women were duped, while Malaysian police say their actions, including washing their hands afterward, showed awareness that they were handling a toxin. Investigators were still examining how VX entered Malaysia and other details of the attack.

VX is an exceptionally toxic, persistent chemical warfare agent banned by the 1993 Chemical Weapons Convention. It can be absorbed through the skin, eyes, lungs or digestive system, and even a small amount can kill. Its oily consistency raised questions about contamination at the airport, medical facilities and other locations. Authorities began decontamination after the identification of VX; no passengers, airport workers or medical staff were reported to have fallen ill. Experts also questioned how the women could have handled the substance and survived.

The killing has caused a diplomatic crisis between Malaysia and North Korea. Pyongyang denies responsibility, disputes Malaysia’s investigation and post-mortem examination, and accuses Malaysia of fabricating evidence under South Korean influence. It has blamed Malaysia for the death and has not confirmed that the victim was Kim Jong-nam. Malaysia, meanwhile, has demanded access to a senior North Korean embassy staff member for questioning and warned the North Korean envoy over his accusations.

Kim Jong-nam was once considered a possible successor to his father, Kim Jong-il, 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 and is believed to possess a substantial chemical arsenal. Outside estimates put its stockpile at 2,500 to 5,000 tons and its production capacity at up to 4,500 metric tons annually in normal conditions. Earlier use of VX included attacks by Japan’s Aum Shinrikyo cult in 1994 and 1995, one of which was fatal.