Federal authorities are also concerned that terrorists may have been planning to deliver a chemical or biological strike from the air using crop-dusters.
Mohamed Atta — the man believed to have piloted one of four hijacked passenger airliners into the North Tower of the World Trade Center — made repeated visits to a crop-dusting airfield in Florida, according to a witness.
Willie Lee, the chief pilot and general manager of South Florida Crop Care in Belle Glade, identified Atta to the FBI, telling agents the man now suspected of being a ringleader among the hijackers came to the airfield as recently as the Saturday before the Sept. 11 attacks, asking questions about crop-dusters, including how big a load of chemicals they could carry.
Atta was "very persistent about wanting to know how much the airplane will haul, how fast it will go, what kind of range it has," Lee told ABCNEWS.
Lee said Atta and as many as 12 or 15 other men appearing to be of Middle Eastern descent visited the airfield in groups of two or three on several weekends prior to the attacks, often taking pictures of the aircraft, which can carry some 500 gallons of solution.
Law enforcement officials have said information about crop-dusting had been downloaded off the Internet by associates of the hijackers and ABCNEWS has confirmed a manual for a crop-duster was found among the belongings of Zacarias Moussaoui, who has been detained since August and is now under arrest as a material witness.
"The FBI assesses the use of this type of aircraft to distribute chemical or biological weapons of mass destruction as potential threats to Americans," Ashcroft told a congressional committee on Monday.
The Federal Aviation Administration lifted a two-day nationwide ban on crop-dusting flights Tuesday morning.