Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, died after being attacked at Kuala Lumpur International Airport on February 13. Malaysian authorities said preliminary toxicology tests found VX nerve agent on swabs from his eyes and face. The killing raised questions about how the banned chemical entered Malaysia, the safety of the airport and North Korea’s possible involvement.

According to investigators, two women approached Kim in the airport’s check-in area and wiped a substance on his face. He sought medical help, suffered convulsions and died on the way to a hospital. Police detained suspects, including the two women, and sought several North Koreans, among them a diplomat and an airline employee. Malaysian authorities said North Korean agents were involved. Relatives and acquaintances of the women, however, maintained that they must have been deceived. Police said at least one woman knew she was handling a toxin and that the women suffered vomiting; toxicologists questioned how they survived exposure.

VX is an extremely potent, oily nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. Even a small amount can be fatal. It is banned under the Chemical Weapons Convention, which North Korea has not signed. Experts said VX can persist on surfaces for days or weeks, prompting concern that the airport, an ambulance or medical facilities might have been contaminated. Malaysian authorities said they would decontaminate areas of the airport visited by the suspects. No passengers, airport workers or medical staff who treated Kim were reported to have fallen ill.

The death triggered a diplomatic dispute. North Korea denied responsibility, accused Malaysia of fabricating evidence and criticized its investigation and insistence on a post-mortem examination. Pyongyang had not confirmed that the dead man, who carried a passport in the name Kim Chol, was Kim Jong-nam. Malaysia warned that the North Korean ambassador could be expelled over his statements about the case.

Kim Jong-nam was the eldest son of former leader Kim Jong-il and was once considered a possible successor. He later lived abroad and criticized North Korea’s policies and hereditary succession. The passages cite estimates that North Korea holds thousands of tons of chemical weapons and has substantial production capacity. They also note an earlier VX attack by the Japanese cult Aum Shinrikyo in the 1990s.