At the time, people were thinking that terrorists might use crop dusters to spread chemical or biological agents. Maybe, but then maybe they had something simplier in mind: gasoline.

A crop duster is a flying tanker. Crashing a gasoline laden crop duster into a building or crowded stadium could kill a lot of people. According to the article, gasoline mixed with air has 15 times the explosive energy as TNT. There is another possibility that wasn't mentioned in the article - spraying the gasoline then igniting it like a fuel-air bomb. It could be tricky (it wouldn't be nice if the engine exhaust prematurely ignited the gasoline spray) but with a little work it could be done.