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 International Airport on February 13, Malaysian authorities said. Preliminary tests detected VX on his face and eyes after two women approached him in the check-in hall and wiped a substance on his face. He sought help, suffered convulsions and died on the way to a hospital.

Kim, the eldest son of former North Korean leader Kim Jong-il, had once been considered a possible successor but lived abroad after falling out of favor. He had criticized North Korea’s dynastic succession and policies. He was travelling under the name Kim Chol and reportedly was awaiting a flight to Macau, where he lived with his family.

Malaysian investigators said North Korean agents orchestrated the attack. They detained two women, one Vietnamese and one Indonesian, and sought several North Koreans, including an embassy official and an airline employee. Police said the women knew they were handling a toxic substance and had been instructed to wash their hands afterward; relatives and acquaintances insisted the women had been deceived. One suspect reportedly suffered vomiting. Toxicologists questioned how the attackers could have handled such a potent agent and survived.

VX is an odorless, oily nerve agent that can penetrate the skin and kill in tiny quantities. It can persist in the environment for days or longer, raising concerns about contamination at the airport and in places involved in Kim’s medical care. Authorities said they would decontaminate areas visited by the suspects; no passengers, airport workers or medical personnel who treated Kim had reported becoming ill.

North Korea denied involvement, criticized Malaysia’s investigation and blamed Malaysia for the death of its citizen. It did not acknowledge that the dead man was Kim Jong-nam. The dispute strained relations between the two countries, and Malaysia warned that it could expel the North Korean ambassador over his accusations.

VX is prohibited under the Chemical Weapons Convention, which North Korea has not signed. Estimates cited in the reports put North Korea’s chemical-weapons stockpile at 2,500 to 5,000 metric tons and its potential annual production capacity at up to 4,500 tons in a typical year, rising to 12,000 tons during an extended crisis. The reports also recalled that Japan’s Aum Shinrikyo cult used homemade VX in attacks on three people in 1994 and 1995, killing one.