institute which north koreans say trades pesticides. equipment such as these silver tanks which experts say are industrial-scale fermenters capable of large scale and biological agents in a sprayable form. andrew weber tracked biological weapons for decades at the pentagon. let's say a thumbnail size of an flax, how many people could that kill with a spray? >> delivered from a spray, tens of thousands of people. >> reporter: from a sprayer in an urban environment? >> absolutely. >> reporter: millions of troops in south korea could be vulnerable to that kind of biological attack. and using biological weapons could give kim jong-un one other advantage. experts say it's much harder to trace who used a biological weapon than it is to trace who used a nuclear weapon.