Washington, D.C. -- Attorney General John Ashcroft said Tuesday that individuals with possible links to the hijackers who attacked New York and the Washington, D.C. area on Sept. 11, 2001, either obtained or attempted to obtain licenses to transport hazardous waste materials.

At least two people detained in the investigations stemming from the Sept. 11 attacks had licenses to transport chemicals and other hazardous materials. And one of the confirmed hijackers visited a Florida crop-dusting company several times in the weeks before the attacks and had sought a government loan for a crop-dusting plane.

The crop-dusting incidents have raised concerns over whether the planes would be used for a biological or chemical attack on the United States. All crop-dusting planes were grounded on Sunday and Monday as a precaution and were back in the air Tuesday.

At the U.S. Truck Driver Training School in Detroit, president Joseph LaBarge told the Associated Press that two men arrested last week in conjunction with the attacks attended the school, and one of them obtained a permit to transport hazardous materials.

The Federal Bureau of Investigations (FBI) has advised people to be on the lookout for anything suspicious related to hazardous materials and crop-dusting aircraft.