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 airport on February 13, Malaysian police said. He was approached by two women in the check-in hall, where a substance was wiped on his face. He sought medical help, suffered convulsions and died on the way to hospital. Malaysian authorities identified him despite his travelling on a passport in the name Kim Chol; North Korea did not acknowledge that the dead man was Kim Jong-nam.

Preliminary Malaysian toxicology tests found VX on swabs from his eyes and face. VX is an extremely potent, persistent chemical warfare agent that can be absorbed through the skin, eyes, lungs or digestive system. Even a tiny amount can be fatal. It is banned under the Chemical Weapons Convention, which North Korea has not signed. The substance’s persistence raised questions about contamination at the airport and potentially at medical facilities and in the ambulance. Authorities said they were decontaminating airport areas visited by the suspects, some 11 days after the attack.

Malaysia said North Korean agents were behind the killing and detained two women, one Vietnamese and one Indonesian, along with a North Korean suspect. Police sought several other North Koreans, including an embassy official and an employee of the state airline. One of the women reportedly vomited after the attack. Investigators said the women had been instructed to wash their hands, while toxicologists questioned how they could have handled VX and survived. Relatives and acquaintances of the women said they believed the suspects had been deceived.

North Korea denied responsibility, challenged Malaysia’s investigation and accused it of fabricating evidence under South Korean influence. A statement attributed to a North Korean legal body blamed Malaysia for the death because it occurred on Malaysian soil and pointed to North Korea’s status as a nuclear weapons state. The dispute strained relations between the countries, and Malaysia warned that the North Korean ambassador could be expelled over his statements.

Kim Jong-nam was the eldest son of former leader Kim Jong-il and had once been considered a possible successor. He lived abroad for years and became a critic of North Korea’s policies and dynastic succession. His death intensified scrutiny of North Korea’s chemical weapons capability. Estimates cited in the reports put its stockpile at 2,500 to 5,000 tonnes and its production capacity at up to 4,500 tonnes in a typical year, rising during a crisis. VX has also been used before: Japan’s Aum Shinrikyo cult employed homemade VX in attacks on three people in 1994 and 1995, killing one.