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’s airport on February 13. Preliminary toxicology tests detected VX on swabs from his eyes and face. Police say two women wiped chemicals on him as he waited for a flight to Macau, where he lived with his family. Kim sought help, suffered convulsions and died on the way to hospital.

VX is an exceptionally toxic nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. It disrupts nerve signals, and even a minute amount can be fatal. The oily substance can persist for days or weeks, raising concerns about contamination at the airport and other locations Kim visited. Authorities said they were beginning decontamination about 11 days after the attack. No passengers, airport workers or medical personnel were reported ill, although police said one of the suspected attackers later vomited.

Malaysian investigators believe North Korean agents organized the killing. They have detained the two women, one Vietnamese and one Indonesian, and are seeking several North Koreans, including an embassy official. Police have suggested the women knew they were handling a toxin because they washed their hands afterward. Relatives and acquaintances insist the women may have been deceived. Experts have also questioned how the attackers avoided more serious exposure, with one suggesting they might have received an antidote; that remains unconfirmed.

North Korea has denied responsibility, criticized Malaysia’s investigation and post-mortem examination, and accused Malaysian authorities of acting under South Korean influence. It has blamed Malaysia for the death and has not confirmed that the victim was Kim Jong-nam, who was travelling under the name Kim Chol. The dispute has strained relations between the two countries, and Malaysia warned the North Korean ambassador that he could be expelled over his accusations.

Kim Jong-nam was once considered a possible successor to his father, Kim Jong-il, but lived abroad for years after falling out of favor. He later criticized North Korea’s policies and dynastic succession.

VX is prohibited under the Chemical Weapons Convention, which North Korea has not signed. Outside estimates put North Korea’s chemical weapons stockpile at 2,500 to 5,000 tons, potentially including VX, and its production capacity at up to 4,500 tons a year under normal conditions. The Japanese cult Aum Shinrikyo previously used homemade VX in attacks on three people in 1994 and 1995, killing one.