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 detected VX nerve agent on swabs from his face and eyes. The killing prompted a criminal investigation, concerns about public safety and a diplomatic dispute between Malaysia and North Korea.

According to investigators, two women approached Kim in the airport 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, became ill and died after suffering convulsions. Malaysian police arrested the two women, one Vietnamese and one Indonesian, along with a North Korean suspect. They also sought several other North Koreans, including an embassy official and an employee of North Korea’s airline. Police said the women had been instructed to wash their hands after the attack and that they had vomited, though relatives and acquaintances maintained that they had been duped.

VX is an exceptionally potent, oily nerve agent that can kill through skin contact in minutes. It can also enter the body through the eyes, by inhalation or ingestion, and can remain dangerous in the environment for an extended period. Its production, stockpiling and use are banned under the Chemical Weapons Convention, which North Korea has not signed. The identification of VX raised questions about possible contamination at the airport and other places Kim had been taken. Malaysian authorities said they were decontaminating areas visited by the suspects; no other airport passengers, workers or medical staff were reported ill.

Malaysia said North Korean agents were responsible and sought to question a senior embassy staff member. Pyongyang denied involvement, criticized Malaysia’s investigation and blamed Malaysian authorities for the death. North Korea described the victim as a North Korean citizen but did not acknowledge that he was Kim Jong-nam, who had been travelling on a passport bearing the name Kim Chol. The dispute strained relations between the two countries, which had previously maintained relatively friendly ties.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, was once considered a potential successor. He lived abroad after falling out of favor and later criticized North Korea’s dynastic succession and policies. North Korea is believed to possess a large chemical weapons stockpile, with estimates cited in the reports ranging from 2,500 to 5,000 tons. VX has previously been used in assassinations: Japan’s Aum Shinrikyo cult attacked three people with homemade VX in 1994 and 1995, killing one.