WASHINGTON — The FBI has questioned more than 3,000 pilots and aircraft owners, most of them in the past year, amid persistent concerns that terrorists might use crop-dusting planes to mount a biological or chemical attack, newly released documents show.
The interviews have not produced any arrests, according to a senior law enforcement official who spoke on condition of anonymity, but have resulted in terrorism investigations that are still under way.
The effort, outlined in documents submitted to the commission investigating the Sept. 11 attacks, is more extensive than previously disclosed and underscores how seriously the threat is viewed by U.S. authorities.
The level of overall terrorist threats against the United States remains extremely high, Attorney General John Ashcroft told reporters Thursday. He echoed the assessments of other top Bush administration officials that the summer months present a number of opportunities for attacks, ranging from the Olympics in Greece to the presidential nominating conventions in Boston and New York.
"There is without a doubt in my mind a very serious level of activity in terrorism which concerns me greatly," Ashcroft said at the Justice Department.
Most of the crop-duster interviews were conducted after the March 2003 start of the war in Iraq, which triggered new concerns about terrorists acquiring and using weapons of mass destruction. The law enforcement official who described the initiative, known as the Agricultural Aviation Threat Project, said it was continuing.
The official declined to provide details, but Andrew Moore, executive director of the National Agricultural Aviation Association, said some of the FBI agents had asked whether any of the pilots and owners knew about attempts by foreigners or interests to purchase crop dusters.
Interrogations of al-Qaida suspects, documents found in Afghanistan and actions by some of the Sept. 11 conspirators pointed to an interest by the terror group in using such planes for an attack.
Mohamed Atta, who piloted one of the planes that struck the World Trade Center, and some associates repeatedly visited a fertilizer company in Belle Glade, Fla., to ask questions about crop dusters, investigators have said. Authorities also found information about crop dusters and dispersal of chemicals on the computer of Zacarias Moussaoui, the only person charged in the United States in connection with the Sept. 11 attacks.
In addition, the capture last year of al-Qaida senior leader Khalid Shaikh Mohammed in Rawalpindi, Pakistan, turned up information on computer hard drives and in handwritten notes about the toxin that causes botulism and about salmonella bacteria and cyanide. Other al-Qaida documents have discussed anthrax and how to make the toxin ricin from castor beans.
The FBI reviewed a list of some 11,000 agricultural aircraft provided by the Federal Aviation Administration, according to documents provided to the Sept. 11 commission. Working from that list, agents interviewed and did background checks on 3,028 operators and owners of the planes.
The initiative was part of a broader FBI effort known as "Operation Tripwire."
that is intended to bolster the intelligence database with a goal of "identifying potential terrorist sleeper cells within the United States" and detecting any pre-attack preparations. The FBI also sought to learn about crop duster capabilities, the law enforcement official said.
Moore said, "We want to do whatever we can to alleviate concerns."
The government has also urged the industry to adopt stringent measures, such as storing aircraft in locked hangars, installing hidden switches to prevent aircraft thefts and securing fertilizer, pesticides and other chemicals.
There remain questions about how successful a crop duster would be for a chemical or biological attack. At a minimum, the dispersal devices would have to be modified, Moore said.
In addition, crop dusters are difficult to fly, are barred from operating in urban areas and would have to be loaded with large quantities of a toxic substance — requiring expertise and the necessary equipment — to be effective, he said.
Crop dusters could be used in attacks on the U.S. food supply, however.
Mark Wheelis, a microbiologist at the University of California-Davis, said in a recent paper on the subject that a large-scale attack on crops or animal herds would require a large stockpile of toxic substances and would entail a great risk of detection or capture.
A smaller-scale attack, however, could have the effect of sowing fear and destabilizing markets even if the actual damage done was rather small, Wheelis said.