Malaysian authorities say Kim Jong-nam, the estranged older half-brother of North Korean leader Kim Jong-un, was killed with the banned nerve agent VX at Kuala Lumpur airport on February 13. Preliminary toxicology tests found the substance on his eyes and face. Kim, who was travelling under the name Kim Chol, sought medical help after two women approached him in the check-in hall, but suffered a seizure and died on the way to hospital.

Malaysian investigators say the women applied chemicals to Kim’s face as North Korean agents watched. One woman later suffered vomiting, police said. Authorities have detained suspects and are seeking several North Koreans, including an embassy official and an employee of the North Korean airline Air Koryo. Police say a suspect’s decision to wash her hands immediately afterward suggests she knew she was handling a toxin. Relatives of the women, however, insist they must have been deceived. Investigators are still examining how VX entered Malaysia and how the attack was carried out.

VX is an exceptionally potent, oily nerve agent that can be absorbed through the skin, eyes, inhalation or ingestion. Even a tiny quantity can be fatal, and the substance can remain hazardous for an extended period. Its discovery raised questions about contamination at the airport and other locations Kim visited, including medical facilities. Malaysian authorities said they were undertaking decontamination; no passengers, airport workers or medical staff were reported to have fallen ill.

The killing has triggered a diplomatic crisis. Malaysia has implicated North Korean agents, while Pyongyang denies involvement, disputes the investigation and blames Malaysia for the death. North Korea has also refused to identify the victim as Kim Jong-nam and has objected to the post-mortem examination. Malaysia’s foreign minister warned the North Korean envoy that he could be expelled over accusations against Malaysian authorities.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, was once regarded as a possible successor but had lived abroad for years. He later criticized North Korea’s policies and hereditary leadership. North Korea has not signed the Chemical Weapons Convention, which bans VX, and outside estimates suggest it maintains a substantial chemical weapons stockpile that may include the agent.