Outside of governments making their own WMDs against international rules, the closest would have to be Aum Shinrikyo. Before their sarin attack in Tokyo, they a

Outside of governments making their own WMDs against international rules, the closest would have to be Aum Shinrikyo. Before their sarin attack in Tokyo, they also manufactured VX gas, recruited scientists from different fields, and conducted experiments in the Australian Outback. There is a more farfetched conspiracy that they used the remoteness to test a nuclear bomb ( https://en.wikipedia.org/wiki/Banjawarn_Station https://en.wikipedia.org/wiki/Banjawarn_Station ).