Malaysian authorities say 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 airport on February 13. Police said preliminary toxicology tests found VX in swabs from his eyes and face. Kim, who lived in Macau and was travelling on a passport bearing the name Kim Chol, sought help after two women approached him in the check-in hall. He suffered convulsions and died on the way to hospital.

Malaysian investigators say the women wiped chemicals on Kim’s face as North Korean agents watched. One suspect became ill and vomited afterward, police said. Authorities were investigating how VX entered Malaysia and seeking several North Koreans, including an embassy official. Relatives and acquaintances of the women said they believed the suspects had been deceived, while police argued that their actions showed awareness that they were handling a toxin.

VX is an exceptionally potent nerve agent that can kill in tiny quantities through skin contact. It disrupts nerve impulses and can cause symptoms including vomiting, seizures and paralysis. Its oily consistency allows it to remain hazardous for an extended period. The discovery raised questions about possible contamination at the airport, medical facilities and the ambulance; authorities said they were beginning decontamination. No airport passengers, workers or medical staff were reported ill. Toxicologists also questioned how the alleged attackers could have handled the substance without more serious effects.

The killing has triggered a diplomatic crisis between Malaysia and North Korea. Malaysian officials say North Korean agents were responsible, while Pyongyang denies involvement, accuses Malaysia of fabricating evidence under South Korean influence, and blames Malaysia for the death. North Korea has not confirmed that the deceased was Kim Jong-nam and objected to Malaysia’s post-mortem examination. Malaysia warned the North Korean ambassador over his allegations.

Kim Jong-nam, the eldest son of former leader Kim Jong-il, was once considered a possible successor 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.

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 the capacity to produce substantially more during a crisis. Previous use of homemade VX was attributed to Japan’s Aum Shinrikyo cult in attacks on three people in 1994 and 1995, one of whom died.