Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, died after being attacked at Kuala Lumpur airport on February 13. Malaysian authorities said preliminary toxicology tests found VX nerve agent on his face and eyes. The killing has raised questions about airport safety and triggered a diplomatic dispute between Malaysia and North Korea.

According to Malaysian investigators, two women approached Kim in the airport’s check-in 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. Police said one of the women later became ill and vomited. Relatives and acquaintances of the suspects said the women may have been deceived into taking part, while Malaysian police maintained that the attackers knew they were handling a toxin.

Malaysia says North Korean agents were involved and has sought several North Koreans for questioning, including an embassy official and an employee of the state airline. Pyongyang denies responsibility, criticizes Malaysia’s investigation and has blamed Malaysia for the death occurring on its territory. North Korea has identified the dead man only as a North Korean citizen; his passport bore the name Kim Chol. The dispute has strained relations between the countries, which previously maintained relatively open ties.

VX is a highly toxic nerve agent banned by the Chemical Weapons Convention. It can be absorbed through the skin and can remain hazardous for an extended period. The discovery prompted concern that the airport and other places Kim had visited might have been contaminated. Malaysian police said they were arranging decontamination and investigating how the substance entered the country. No passengers, airport workers or medical staff were reported to have become ill, though toxicologists questioned how the two suspected attackers avoided more serious exposure.

North Korea has not signed the chemical weapons treaty and is widely believed to possess a substantial chemical arsenal that includes VX. Outside estimates cited in the reports put its stockpile at between 2,500 and 5,000 metric tons and its annual production capacity at up to 4,500 metric tons under typical conditions. VX has also been used in an assassination before: Japan’s Aum Shinrikyo cult used homemade VX in attacks on three people in 1994 and 1995, killing one.

Kim Jong-nam was the eldest son of former North Korean leader Kim Jong-il and had once been considered a potential successor. He lived abroad for years after being passed over in favor of Kim Jong-un and later criticized North Korea’s policies and dynastic succession.