Malaysian authorities say 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 International Airport on February 13. Preliminary toxicology tests detected VX in swabs from his eyes and face. Police are investigating how the substance entered Malaysia and how it was used.

According to investigators, two women approached Kim in the departure hall and wiped chemicals on his face as he waited for a flight to Macau, where he lived with his family. He sought medical help, suffered convulsions and died on the way to hospital. One of the women later experienced vomiting, police said. Malaysian authorities believe the women knew they were handling a toxin, citing their actions after the attack, but relatives and acquaintances have insisted they may have been deceived. Toxicologists have questioned how the women could have survived handling such a powerful substance.

VX is a highly toxic, oily nerve agent that can be absorbed through the skin or eyes, inhaled or ingested. Even a minute quantity can be fatal, and its persistence raised concerns about possible contamination at the airport and other locations connected to the attack. Authorities said they were beginning decontamination, more than 10 days after the killing. No passengers, airport workers or medical staff were reported to have fallen ill.

Malaysia suspects North Korean agents were behind the killing and is seeking several North Koreans, including an embassy official and an employee of the state airline. North Korea denies involvement, has criticized Malaysia’s investigation and post-mortem examination, and says Malaysia bears responsibility because the death occurred there. It has not acknowledged that the dead man, who was carrying a passport in the name Kim Chol, was Kim Jong-nam. The dispute has strained relations between the two countries.

Kim Jong-nam was the eldest son of former North Korean leader Kim Jong-il and was once considered a possible successor. He lived abroad for years after being bypassed in favor of Kim Jong-un and later publicly criticized the North’s policies and hereditary succession.

VX is prohibited under the Chemical Weapons Convention, which North Korea has not signed. Outside estimates put the North’s chemical weapons stockpile at 2,500 to 5,000 tons, with VX among the agents it is believed to possess. A U.S. Congressional Research Service report estimated that North Korea could produce up to 4,500 metric tons of chemical weapons in a typical year and more during an extended crisis.