Malaysian investigators say 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 on February 13. Preliminary toxicology tests detected the substance in swabs from his face and eyes. Kim, who was traveling on a passport under the name Kim Chol, sought help after two women approached him and applied a substance to his face. He suffered seizures and died on the way to a hospital.

Malaysia has accused North Korean agents of orchestrating the attack and detained suspects, including the two women, one Vietnamese and one Indonesian. Investigators are seeking several North Koreans, among them an embassy official and an employee of the state airline Air Koryo. Police say the women knew they were handling a toxin, citing their efforts to wash their hands afterward. One reportedly became ill and vomited. Relatives and acquaintances, however, insist the women may have been deceived into participating. Toxicologists have questioned how the attackers could have handled VX without more severe effects.

VX is an exceptionally potent, oily nerve agent that can enter the body through the skin, eyes, inhalation or ingestion. Tiny amounts can be fatal, and the substance can remain dangerous in the environment for an extended period. Malaysia began decontaminating the airport about 11 days after the attack and said it intended to address other locations visited by the suspects. No passengers, airport workers or medical staff were reported ill in the immediate aftermath.

The killing has triggered a diplomatic dispute. North Korea denies responsibility, has criticized Malaysia’s investigation and post-mortem examination, and blames Malaysia for the death. It has not acknowledged that the victim was Kim Jong-nam or indicated that it would allow the embassy official to be questioned. Malaysia warned North Korea’s ambassador that he could be expelled over his accusations.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, had once been considered a possible successor but lived abroad for years and became a critic of his family’s dynastic rule. North Korea has not signed the Chemical Weapons Convention, which bans VX. Outside estimates put its chemical weapons stockpile at 2,500 to 5,000 tons, with VX reportedly among the agents it possesses.