BANGKOK—One by one, on different flights at different times, four men from North Korea flew into Malaysia over the course of a fortnight in February, coming together finally in a condo apartment on Kuchai Lama Street in a bustling middle-class suburb on the edge of the country’s sprawling capital, Kuala Lumpur. According to multiple reports, after the murder and at the height of the scandal, they huddled frequently with Ri Jong Chol, a 47-year-old North Korean who holds a science degree and who moved to Malaysia in August 2016.

Ri worked for an outfit called Tombo Enterprises. According to its website it makes anti-cancer supplements and emphasizes “wholesome treatment” built around herbal medicines. But Ri may have been more useful to the new arrivals for a particular set of skills: handling and combining the binary components of the deadly nerve agent VX, the weapon of mass destruction that would be used to kill Kim Jong Nam, estranged older half-brother of North Korean dictator Kim Jong Un.

VX, which is outlawed around the world by signatories of the Chemical Weapons Convention, is a slightly yellow liquid with no smell and no taste. When it’s dispersed as an aerosol, victims do not know they have been affected until they begin to die, quickly and horribly, more or less like flies zapped with bug spray, choking, twisting, and kicking as their nervous system shuts down. In one infamous incident at the Dugway Proving Grounds in Utah in 1968 more than 4,000 sheep were killed by accident some 30 miles from the test site.