Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, died after being attacked with the nerve agent VX at Kuala Lumpur International Airport on February 13. Malaysian authorities said traces of the banned chemical were found in swabs from his face and eyes. He sought help after two women approached him, but suffered convulsions and died on the way to a hospital.

Malaysian investigators said the women wiped chemicals on Kim’s face while North Korean agents looked on. Police detained the two women, one Vietnamese and one Indonesian, and sought several North Koreans, including an embassy official. One of the women reportedly vomited after the attack. Relatives and acquaintances said the women may have been duped, while police argued that their actions, including washing their hands afterward, indicated they knew they were handling a toxin. Investigators were still trying to establish how VX entered Malaysia.

VX is a highly toxic, persistent nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. A tiny amount can be fatal, and contaminated surfaces may remain dangerous for an extended period. Its discovery raised questions about safety at the airport, which had not been decontaminated for 11 days after the attack. Authorities said they would decontaminate the airport and other locations connected to the suspects. No airport passengers, workers or medical staff who treated Kim were reported ill.

The killing triggered a diplomatic dispute. Malaysia said North Korean agents were responsible, while Pyongyang denied involvement, accused Malaysia of fabricating evidence and objected to the post-mortem examination. North Korea did not acknowledge that the dead man, who was travelling on a passport in the name Kim Chol, was Kim Jong-nam. His body remained in a Malaysian hospital mortuary amid disagreement over who should claim it.

Kim Jong-nam was the eldest son of former North Korean leader Kim Jong-il and had once been considered a possible successor. He lived abroad for years after falling out of favour and later criticized the North’s political system and dynastic succession.

VX is banned under the Chemical Weapons Convention, which North Korea has not signed. Outside estimates cited in the reports put its chemical weapons stockpile at 2,500 to 5,000 tonnes and its production capacity at up to 4,500 tonnes in a typical year. The substance has been used before: members of Japan’s Aum Shinrikyo cult used homemade VX in attacks on three people in 1994 and 1995, killing one.