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 International Airport, Malaysian police said. Preliminary toxicology tests detected the substance on his eyes and face after two women approached him in the check-in hall on February 13. Kim sought medical help, suffered convulsions and died on the way to hospital.

Malaysian investigators say the women applied chemicals to Kim’s face while North Korean agents looked on. One of the women, who are Vietnamese and Indonesian, later suffered vomiting, police said. Relatives and acquaintances insist the women were duped, while police say their actions, including washing their hands immediately afterward, indicate they knew they were handling a toxin. Investigators were still trying to determine how VX entered Malaysia and were seeking several North Koreans, including an embassy official.

VX is an exceptionally potent, persistent nerve agent that can be absorbed through the skin. Its discovery raised questions about how the attackers avoided serious harm and whether the airport, ambulance or medical facilities had been contaminated. Authorities began decontamination work after the airport had remained open for 11 days. No passengers, airport workers or medical staff were reported to have fallen ill.

The killing has caused a diplomatic crisis between Malaysia and North Korea. Malaysia suspects North Korean involvement; Pyongyang denies responsibility, has criticized the investigation and blamed Malaysia for the death. North Korea has also refused to identify the dead man as Kim Jong-nam, who was travelling on a passport in the name Kim Chol. His body remained in a hospital mortuary amid a dispute over its release.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, was once considered a possible successor but had lived abroad for years and publicly criticized North Korea’s dynastic succession and policies. North Korea has not signed the Chemical Weapons Convention and is believed to possess a large chemical weapons stockpile, with outside estimates putting it at 2,500 to 5,000 tons.