The discovery has added to concerns among government counterterrorism experts that the bin Laden conspirators may have been planning — or may still be planning —to disperse biological or chemical agents from a cropdusting plane normally used for agricultural purposes.
Among the belongings of suspected terrorist Zacarias Moussaoui, sources tell TIME, were manuals showing how to operate cropdusting equipment that could be used to spray fast-killing toxins into the air.
The discovery resulted in the grounding of all cropdusters nationwide on Sunday Sept. 16th. The dusters have been allowed back up, but are not allowed to take off or land from what traffic controllers refer to as Class B airspace, or the skies around major cities.
One senior official cautions that because corroborative evidence is lacking the FBI does not place "high credibility" in the notion that the hijackers were in fact exploring the idea of stealing or renting cropdusters. However, the FBI is advising members of a crop-dusters' group to report any suspicious buys of dangerous chemicals in the wake of last week's terrorist attacks.
Last week, the National Agricultural Aviation Association, a crop dusters trade group, posted a message from the FBI to its membership: "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 behavior by employees or customers, and unusual contacts with the public. Members should report any suspicious circumstances or information to local FBI offices."
—With reporting by Elaine Shannon/Washington