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, Malaysian authorities said. Preliminary toxicology tests found the substance in swabs from his eyes and face. He sought help after two women approached him, but suffered convulsions and died on the way to hospital.

Malaysian investigators say the women wiped chemicals on Kim’s face while North Korean agents watched. One woman, who is Vietnamese, and the other, who is Indonesian, were taken into custody; police said one later suffered vomiting. Relatives and acquaintances have suggested that the women were duped, while Malaysian police maintain that at least one knew she was handling a toxin because she washed her hands immediately afterward. Authorities were seeking several North Koreans, including an embassy official, and investigating how VX entered Malaysia.

The identification of VX raised concerns about contamination at the busy airport, which had gone 11 days without decontamination. The oily, odorless agent can penetrate the skin, remain lethal for long periods and kill in minute quantities. Officials said they would decontaminate the airport and places visited by the suspects. No passengers, airport workers or medical staff were reported to have fallen ill. Toxicologists also questioned how the women could have handled VX and survived.

The killing triggered a diplomatic dispute. North Korea denied responsibility, criticized Malaysia’s investigation and post-mortem examination, and blamed Malaysia for the death. It did not confirm that the dead man was Kim Jong-nam, who had been traveling under the name Kim Chol. Malaysia, meanwhile, accused North Korean agents of carrying out the attack and warned the North Korean ambassador over his public statements.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, had once been considered a possible successor. He lived abroad for years, including in Macau, and became a critic of his family’s dynastic rule.

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