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’s airport, Malaysian police said. A preliminary chemical analysis found traces of VX on his eyes and face after two women allegedly wiped chemicals on him in the check-in hall on February 13. Kim sought help from airport staff, then suffered convulsions and died on the way to a hospital.

Kim had lived abroad for years and was travelling under the name Kim Chol. Once considered a possible successor to his father, former North Korean leader Kim Jong-il, he had become a prominent critic of the regime and its dynastic succession. Malaysian authorities say North Korean agents were behind the attack. They detained the two women, one Vietnamese and one Indonesian, and sought several North Koreans for questioning, including an embassy official. Relatives of the women said they believed the suspects had been deceived, while police argued that at least one woman’s actions showed awareness that she had handled a toxin. One of the women reportedly vomited after the attack.

VX is an exceptionally potent, oily nerve agent that can enter the body through the skin, eyes, lungs or digestive system. Even a tiny quantity can be fatal, and its persistence raised concerns about contamination at the airport and in medical facilities. Malaysian authorities said they were beginning decontamination; no passengers, airport workers or medical staff were reported to have fallen ill.

The killing triggered a diplomatic dispute. North Korea denied responsibility, challenged Malaysia’s investigation and post-mortem examination, and blamed Malaysia for the death. It also declined to identify the victim as Kim Jong-nam. Malaysia warned North Korea’s ambassador that he could be expelled over his accusations.

VX is banned under the Chemical Weapons Convention, which North Korea has not signed. Outside estimates cited in the reports put North Korea’s chemical weapons stockpile at 2,500 to 5,000 tonnes and suggest substantial production capacity, although its precise holdings are uncertain. The Japanese cult Aum Shinrikyo previously used homemade VX in attacks in the 1990s, killing one person.