Malaysian authorities say Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, was killed with the banned VX nerve agent at Kuala Lumpur airport on February 13. Toxicology tests detected VX on his eyes and face after two women allegedly approached him and wiped chemicals on his face. Kim sought help, suffered convulsions and died on the way to hospital. He was carrying a passport under the name Kim Chol.

Malaysia suspects North Korean agents directed the attack and has detained the two women, one Vietnamese and one Indonesian, while seeking several North Koreans, including an embassy official. Police say the women knew they were handling a toxin, citing their instructions to wash their hands afterward. Relatives and acquaintances contend the women may have been deceived. One of the women reportedly suffered vomiting after the attack. Toxicologists have questioned how the attackers avoided more serious exposure; one suggested they might have received an antidote, though this has not been established.

VX is a highly toxic, oily nerve agent that can enter the body through the skin, eyes, lungs or digestive system. Tiny quantities can be fatal, and its persistence raised concerns about possible contamination at the airport, in the ambulance and at medical facilities. Authorities began decontamination after the substance was identified, roughly 11 days after the attack. No passengers, airport workers or medical personnel were reported to have become ill.

The killing has triggered a diplomatic dispute. North Korea denies responsibility, criticizes Malaysia’s investigation and post-mortem, and blames Malaysia for the death. It has not acknowledged that the victim was Kim Jong-nam and has given no indication it will comply with Malaysia’s request to question an embassy staff member. Malaysia warned the North Korean ambassador that he could be expelled over his accusations.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, was once considered a possible successor but had lived abroad for years and criticized North Korea’s dynastic succession and policies. North Korea has not signed the Chemical Weapons Convention and is believed to possess a substantial chemical weapons stockpile, potentially including VX. The agent has been used before: members of Japan’s Aum Shinrikyo cult attacked three people with homemade VX in 1994 and 1995, killing one.