The airport worker admits he had been puzzled. It was back in February and the small airfield at Belle Glade, Florida, surrounded by sugar-cane fields, was visited by three Middle Eastern men who wanted to know about crop dusters, how they worked and their capacity.
The men returned once and there were other Middle Eastern visitors with similar questions. They took pictures and a video. The leader of the first group, he is now sure, was Mohamed Atta (33), who flew one of the planes into the World Trade Centre and who is believed to have been the leader of the terrorist teams that attacked the US.
Police also found a manual on crop dusters on another man, Zacarias Moussaoui, held for questioning in August. He has been refusing to co-operate with the FBI but has been linked to Osama bin Laden associates.
The discoveries have raised the chilling possibility that bin Laden's teams were investigating the possibility of using crop dusters for a chemical or biological attack. Perhaps, some are saying, that is still on the agenda and crop-duster flights have been severely restricted. The head of the CIA, Mr George Tenet, last year, after all, warned in Congressional testimony that supporters of bin Laden "had trained to conduct attacks with toxic chemical or biological toxins".
Indeed, the possibility that the World Trade Centre attack also involved biological weapons was taken very seriously. Four hours after the attack the Centre for Disease Control (CDC) issued a health alert asking hospitals to monitor for any symptoms that might reflect a biological attack. Specialist teams were sent to the site to carry out tests.
Although biological weapons are difficult to produce or procure, a biological attack using a small plane would in reality be easier to organise, particularly if the crew was willing to die, than the sophisticated attacks mounted this time.
A small cloud of bacteria could silently infect thousands with any of dozens of deadly diseases from anthrax, to smallpox, to pneumonic plague, with those infected passing the diseases on to others before anyone was even aware of an attack. With many taking days to show symptoms, such an attack has a very deadly potential in a country realising it is ill-prepared for such a form of warfare and whose doctors would take some time to recognise an outbreak.
Mr Jerome Hauer, former head of New York City's emergency-preparedness office has warned that "the nation is woefully unprepared to deal with bioterrorism".
In a federal exercise three months ago, 24 simulated cases of smallpox were "discovered" in US hospitals as part of an assessment of US bioterrorism preparedness. Less than two weeks after those cases popped up, computer models indicated that - if the exercise had been real - 15,000 people would have contracted the disease and 1,000 would have died.
The "epidemic" was still raging when the exercise ended and, the computer models predicted, rioting and looting would have broken out as vaccine supplies ran out.
Yet the US has barely enough smallpox vaccine, 7.5 million units, to cope with a medium-sized attack. Orders put in last year for 40 million more will not be available until 2004. Of some 50 deadly bacteria believed usable in such attacks, vaccines exists for barely a dozen.
"If we had a serious chemical or biological attack, we wouldn't be able to send large numbers of rescuers in with protective suits," says Mr Bob Confide of LA's emergency-preparedness office. We only have limited numbers of decontamination suits. They could be counted on both hands.
The events in New York and Washington were tragedies beyond what anyone had previously imagined, but the potential of biological terrorism is far greater in terms of loss of life and disruption," Dr Michael Stromal, director of the University of Minnesota's Centre for Infectious Disease Research and Policy, told the Washington Post. It would be less graphic - no flames and explosions - but much more insidious. Anyone with a cough would be a weapon."
A January 2001 report by the CD in Atlanta concluded that the nation's public-health infrastructure is not adequate to detect and respond to a bilateralistic event. Another report warned that a fifth of the nation's vaccine supplies were stored in temperatures that were too high. Some improvements have been implemented since then.
An international biological weapons convention signed by 143 nations has outlawed the development, production and stockpiling of biological weapons since 1975, but the absence of any formal verification regime to monitor compliance has limited the effectiveness of the convention. And, specifically, there are fears that both Iraq and Libya have developed stocks.
Bioterrorism is not new. Fourteenth-century barbarians tossed plague-infected corpses over the walls of fortified cities to spread infection among their enemies. Recently, in the mid-1990S, UN weapons inspectors discovered that Iraq had stockpiled warheads containing anthrax spores and the toxin that causes botulism.