Malaysian authorities say Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, was killed with the highly toxic 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, sought help after a woman wiped something on his face. He suffered convulsions and died on the way to hospital.

Malaysian investigators say two women, one Vietnamese and one Indonesian, applied the poison while North Korean agents were involved. One of the women later vomited, according to police, who are investigating how the substance entered Malaysia. Relatives and acquaintances of the women say they may have been deceived; Malaysian authorities dispute that account. Several suspects were in custody, and police sought other North Koreans, including an embassy official. Toxicologists questioned how the alleged attackers could have handled VX without more severe effects.

VX is a potent chemical warfare agent banned by the Chemical Weapons Convention. It can penetrate the skin and disrupt the nervous system, and even a tiny quantity can be fatal. Because it can persist for days or longer, its discovery raised concerns about contamination at the airport and other locations Kim visited, including medical facilities and an ambulance. Authorities said they were decontaminating the airport and places linked to the suspects. No airport passengers, workers or medical staff were reported to have fallen ill.

The killing has intensified a diplomatic dispute between Malaysia and North Korea. Pyongyang denies responsibility, has criticized Malaysia’s investigation and post-mortem examination, and says Malaysia bears responsibility for the death on its territory. North Korea has not confirmed that the dead man was Kim Jong-nam. Malaysia has warned the North Korean ambassador over his accusations and sought to question an embassy official.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, had once been considered a possible successor but lived abroad for years after being passed over in favour of his younger half-brother. He later criticized North Korea’s policies and dynastic succession.

North Korea has not signed the Chemical Weapons Convention and is widely believed to possess a large chemical weapons stockpile. Outside estimates put its inventory at 2,500 to 5,000 tonnes and its potential annual production capacity at 4,500 tonnes in normal conditions, rising during a crisis. Previous use of homemade VX was reported in Japan in 1994 and 1995, when members of the Aum Shinrikyo cult attacked three people, killing one.