Months before killing about a dozen commuters and severely injuring dozens more in Tokyo with sarin, another kind of nerve gas, in March 1995, the Aum Shinrikyo cult tried VX on at least three victims, killing one whom cult members believed was a police informant.
In their trial, cult members said they practiced using syringes to spray the deadly chemical on people's necks as they pretended to be out jogging. The suspected police informant spent 10 days in a coma before dying.
He was walking on the sidewalk in his neighborhood in Tokyo in January 1995 when a member of the cult sprayed the nerve agent on the back of his neck. Most of it was blocked by his jacket collar.
"I had no idea what happened at that time," he said. He was attacked because he was a vocal opponent of the cult.
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He finished walking home but about half an hour later realised everything seemed to be oddly dark - an effect of the toxin causing his pupils to shrink. He started feeling hot inside and, sweating profusely, took off his clothes.
He was rushed to a hospital for emergency treatment, and was unconscious for several days.
"I was saved by the collar of the jacket I was wearing," he told NHK after officials in Malaysia said they suspected VX in the Kuala Lumpur killing.
According to court documents, two trained chemists who had joined the cult developed VX in a customized lab in the summer of 1994. They initially wanted to produce 1 kilogram (2.2 pounds) but succeeded in making only about 70 grams (2.5 ounces).
More than 20 years after the attack, Nagaoka still has numbness on the right side of his body and uses an oxygen tube inserted in his nostrils to assist his breathing.
Nagaoka said when he saw Kim Jong Nam in an airport surveillance video walking unassisted for a while but gradually seeming to slow down, he thought it must be VX.
He also said Kim might have been sweating heavily like he did, citing wet spots on Kim's shirt when he was shown slumped in a chair.