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 International Airport on February 13. A preliminary toxicology analysis found the substance in swabs from his eyes and face. Police are investigating how it entered Malaysia and who organized the attack.

Kim was waiting for a flight to Macau, where he lived with his family, when two women allegedly approached him and wiped chemicals on his face. He sought help from airport staff, saying someone had splashed or sprayed him with liquid, but suffered convulsions and died on the way to hospital. He was carrying a passport in the name Kim Chol. North Korea has not confirmed his identity.

Malaysian police have detained the two women, one Vietnamese and one Indonesian, and are seeking several North Koreans, including an embassy official and an employee of the state airline. Police say the women knew they were handling a toxin, citing the fact that one washed her hands immediately afterward. Relatives and acquaintances insist the women were deceived. Police also said one of the women later suffered vomiting. Toxicologists have questioned how the attackers could have handled VX without more severe effects.

VX is a highly toxic, oily nerve agent banned under the Chemical Weapons Convention. It can be absorbed through the skin and remains hazardous for a prolonged period, raising concerns about contamination at the airport, in the ambulance and at medical facilities. Malaysian authorities said they were beginning decontamination about 11 days after the attack. No passengers, airport workers or medical staff were reported to have fallen ill.

The killing has triggered a diplomatic dispute. Malaysia suspects North Korean agents were responsible; Pyongyang denies involvement, accuses Malaysia of fabricating evidence under South Korean influence and says Malaysia bears responsibility because the death occurred on its soil. Malaysia has demanded access to an embassy official for questioning and warned the North Korean ambassador over his accusations.

Kim Jong-nam was once seen as a possible successor to his father, Kim Jong-il, but lived abroad after falling out of favor and became a critic of the North Korean regime. North Korea has not signed the chemical weapons treaty and is believed to possess a substantial chemical arsenal, including VX. The agent has previously been used in attacks by Japan’s Aum Shinrikyo cult.