Kim Jong-nam, the estranged half brother of North Korean leader Kim Jong-un, was killed with the highly toxic VX nerve agent at Kuala Lumpur International Airport, Malaysian police said. A preliminary toxicology analysis detected VX in swabs from his eyes and face after the February 13 attack.

According to Malaysian investigators, two women approached Kim in the airport check-in hall and wiped chemicals on his face as North Korean agents looked on. Kim sought help, saying liquid had been splashed or sprayed on him, but suffered convulsions and died on the way to a hospital. Police said one of the women later vomited. The two women, one Vietnamese and one Indonesian, were detained; relatives and acquaintances said they believed the women had been tricked. Police maintained that at least one woman knew she was handling a toxin, citing her immediate handwashing. Authorities were also seeking several North Koreans, including an embassy official, and investigating how VX entered Malaysia.

VX is an exceptionally potent, persistent nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. Even a tiny amount can be fatal. Its use raised questions about contamination because the airport had gone 11 days without decontamination. Authorities said they were beginning to clean the airport and other locations associated with the case. No passengers, airport workers or medical staff who treated Kim had reportedly fallen ill.

The killing triggered a diplomatic dispute between Malaysia and North Korea. Pyongyang denied responsibility, criticized Malaysia’s investigation and post-mortem examination, and blamed Malaysia for the death. It also declined to identify the victim as Kim Jong-nam, who was travelling on a passport bearing the name Kim Chol. Malaysia warned the North Korean ambassador that he could be expelled over his accusations.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, had once been considered a possible successor but lived abroad for years after being passed over in favor of Kim Jong-un. He had publicly criticized North Korea’s policies and dynastic 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 tons and its annual production capacity at up to 4,500 metric tons in ordinary conditions. The Japanese cult Aum Shinrikyo previously used homemade VX in attacks on three people in 1994 and 1995, killing one.