Kim Jong-nam, the estranged half brother of North Korean leader Kim Jong-un, was killed with the banned VX nerve agent at Kuala Lumpur International Airport on February 13, Malaysian police said. Toxicology tests detected VX in swabs from his eyes and face. The killing has raised questions about public safety and triggered a diplomatic dispute between Malaysia and North Korea.

Malaysian investigators say two women, one Vietnamese and one Indonesian, applied chemicals to Kim’s face as he waited for a flight to Macau, where he lived with his family. He sought help from airport staff, saying a liquid had been splashed or sprayed on him, but suffered convulsions and died on the way to a hospital. One of the women later experienced vomiting, police said. Authorities are investigating how the substance entered Malaysia and are seeking several North Koreans, including an embassy official. Relatives and acquaintances of the women contend that they were duped, while Malaysian police say the attackers knew they were handling a toxin.

VX is a highly toxic nerve agent that can be absorbed through the skin, eyes, lungs or digestive system. Even a small amount can kill, and the oily substance can remain dangerous for an extended period. Its use, production and stockpiling are banned under the Chemical Weapons Convention. The discovery prompted concern that the airport and places Kim visited, including medical facilities and an ambulance, might have been contaminated. Malaysian authorities said they were beginning decontamination; no passengers, airport workers or medical staff were reported ill.

Suspicion has focused on North Korea, which Malaysia says was behind the attack. Pyongyang denies responsibility, accuses Malaysia of fabricating evidence under South Korean influence and blames Malaysia for the death. It has also refused to identify the victim as Kim Jong-nam, who was travelling on a passport bearing the name Kim Chol. The dispute has strained relations between the two countries and led to disagreements over the investigation and custody of his body.

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 bypassed in favor of Kim Jong-un. He later criticized North Korea’s policies and hereditary succession.

North Korea has not signed the Chemical Weapons Convention and is widely believed to possess a large chemical weapons stockpile, estimated at 2,500 to 5,000 tons. Outside estimates put its production capacity at up to 4,500 metric tons in a typical year and 12,000 tons during an extended crisis. Earlier, in 1994 and 1995, Japan’s Aum Shinrikyo cult used homemade VX in attacks on three people, killing one.