There were several news articles around that time that wrote about the possibilty of terrorists arming planes with explosives, possibly nukes or bio-weapons, and crashing them in the US.

In 2000, I had an interview at Dean Witter at WTC and I called my wife afterward and said to her that if they made me any kind of offer, I'd turn them all down. She asked "why?" and I told her because if the terrorists that attempted to bring down the WTC in 1993 and failed, they'd try it again and would succeed. - and I told her this not because I was being prophetic, the very idea was in the news of the day back then and in commentaries in the media, that this is what terrorists were planning for America.