Kim Jong-nam, the estranged half brother of North Korean leader Kim Jong-un, was killed with the highly toxic nerve agent VX at Kuala Lumpur International Airport on February 13, Malaysian police said. A preliminary chemical analysis detected the substance on swabs from his eyes and face. Kim, who was travelling under the name Kim Chol, sought help after two women allegedly wiped a substance on his face. He suffered convulsions and died on the way to hospital.

Malaysian investigators believe North Korean agents orchestrated the attack and are seeking several North Koreans, including an embassy official and an employee of the state airline. Two women, one Vietnamese and one Indonesian, were detained. Police said one later suffered vomiting and argued that handwashing immediately after the attack suggested the women knew they were handling a toxin. Relatives and acquaintances maintained that the women must have been deceived. Toxicologists also questioned how the attackers could have handled VX without more severe effects.

The finding raised concerns about contamination at the airport and other locations, including the ambulance and medical facilities. VX is an oily, persistent nerve agent that can be lethal in minute amounts and can enter the body through the skin, eyes, inhalation or ingestion. Malaysian authorities said they were beginning decontamination; no airport passengers, workers or medical staff were reported ill.

North Korea denied involvement, criticized Malaysia’s investigation and post-mortem examination, and blamed Malaysia for the death. It also refused to confirm that the dead man was Kim Jong-nam, intensifying a diplomatic dispute with a country that had previously maintained relatively accessible ties with Pyongyang.

Kim Jong-nam was the eldest son of former leader Kim Jong-il and had once been considered a possible successor. He had lived abroad for years after being passed over in favour of Kim Jong-un and had publicly criticized North Korea’s political system and hereditary succession.

VX is banned under the Chemical Weapons Convention, which North Korea has not signed. Outside estimates put its chemical weapons stockpile at 2,500 to 5,000 tonnes, with VX among the agents it is believed to possess. North Korea is also estimated to be capable of producing large quantities of chemical weapons. The Japanese cult Aum Shinrikyo previously used homemade VX in attacks on three people in 1994 and 1995, killing one.