A manual on cropdusting has been found in a suspected terrorist hideout - raising the spectre that they were planning to use chemical or biological weapons.
Time magazine reports that the finds by US authorities led to all cropdusting planes being grounded amid fears the terror gang could use them to spray nerve agents or other chemicals.
The planes have since been allowed to fly again, but are not allowed near major cities, and the Federal Aviation Authority has banned flights over major sporting events or tonight's prayer service at the New York Yankees stadium, which is expected to attract more than 60,000 people.
According to Time, FBI agents have asked cropdusters to report any unusual or suspicious purchases of dangerous chemicals.
American agents found the manual while searching a suspected terrorist hideout as part of their investigation into the World Trade Centre and Pentagon terror strikes, but they have refused to say which city or state it was in.
A senior official told Time there was no corroborative evidence to support the theory that terrorists were planning to steal or rent cropdusting planes.
But the National Agricultural Aviation Association issued a message stating: "Members should be vigilant to any suspicious activity relative to the use, training in or acquisition of dangerous chemicals or airborne application of same, including threats, unusual purchases, suspicious behaviour by employees or customers, and unusual contacts with the public."
The find came as officials from the US government and the airline industry disclosed that "knife-like weapons" were found on other passenger aircraft on September 11th, the day of the terrorist hijackings.
The weapons were found on two separate Delta Airlines planes, both of which were grounded after the terror attacks, suggesting that other hijackings could have been planned.
Another weapon was discovered on a plane belonging to a different carrier, meaning the terror gang could have infiltrated four airlines. United and American Airlines planes were targeted in the hijacks.