Kim Jong-nam, the estranged older 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 detected VX nerve agent on his face and eyes. The killing has prompted an investigation, raised concerns about public safety and triggered a diplomatic dispute between Malaysia and North Korea.

According to Malaysian police, 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. One of the women later experienced vomiting, police said. Both women, one Vietnamese and one Indonesian, were taken into custody. Relatives and acquaintances have said they believe the women were deceived, while Malaysian authorities maintain that they knew they were handling a toxin. Police were also seeking several North Koreans, including an embassy official, and investigating how the substance entered Malaysia.

VX is a highly toxic nerve agent banned under the 1993 Chemical Weapons Convention. It can be absorbed through the skin or eyes, inhaled or ingested, and even a small amount can be fatal. Because the oily substance can remain dangerous for an extended period, its discovery raised questions about contamination at the airport and other places Kim had visited. Authorities said they were decontaminating the airport and areas associated with the suspects. No passengers, airport workers or medical staff who treated Kim had reportedly become ill.

Malaysia has said North Korean agents were responsible for the attack, an allegation Pyongyang strongly denied. North Korea accused Malaysian authorities of fabricating evidence under South Korean influence and blamed Malaysia for the death. It objected to the post-mortem examination and had not acknowledged that the victim, who carried a passport in the name Kim Chol, was Kim Jong-nam. Malaysia warned the North Korean ambassador that he could be expelled over his accusations.

Kim Jong-nam was once considered a possible successor to his father, Kim Jong-il, but lived abroad for years after being passed over in favor of Kim Jong-un. He later criticized North Korea’s policies and dynastic succession. North Korea has not signed the Chemical Weapons Convention and is widely believed to possess a substantial chemical weapons stockpile, although the passages do not establish the source of the VX used in the attack.