New advances in technology have made it possible for terrorists to kill millions of people with chemical or biological weapons, the World Health Organisation warned yesterday.
In a draft of a 179-page report that was rushed out after calls for advice on how to combat germ warfare, the United Nations health agency said: "The magnitude of possible impacts on civilian populations of their use or threatened use obliges governments both to seek prevention and to prepare response plans."
The WHO's report was issued as it emerged that an order would be issued in the United States today grounding all crop-dusting planes in response to fears that terrorists may have been plotting to use the aircraft to spray chemical or biological weapons.
A Federal Aviation Authority source said that "national security agencies" had asked the authority to ground the 3,500 crop-dusters, amid growing evidence that the terrorists behind the 11 September attacks had plans to use them to launch other attacks.
A flight manual for crop-dusters was found among the possessions of Zacarias Moussaoui, who is in federal custody on immigration violations. He was detained after he sought flight training in Minnesota and the school grew suspicious and called authorities.
Yesterday it was revealed that a group of Middle Eastern men, apparently including Mohamed Atta – the man believed to have flown a hijacked airliner into the north tower of the World Trade Centre in New York – had made numerous inquiries about crop-dusters at a Florida fertiliser company in recent months.
J D "Will" Lee, the general manager of South Florida Crop Care in Belle Glade, said he was visited by two or three men of Middle Eastern origin almost every weekend for six to eight weeks before the attacks in New York and Washington. He said they were persistent and asked "odd questions" about his blue and yellow 502 Air Tractor crop-duster.
"I wouldn't spend any time talking to them or telling them anything because I didn't think it was any of their business," said Mr Lee.
Often arriving in rented vans at the Belle Glade municipal airport, where the crop-dusting business is located, the men asked about the range of the aircraft, what quantity of chemicals it would haul, how difficult it was to fly and how much fuel it would carry, he said.
Mr Lee said that a colleague, James Lester, had identified one of the men to the FBI as Mohamed Atta.
Mr Lester told The Washington Post that Atta repeatedly asked him to let him see the interior of the cockpit and asked how to start the planes. Mr Lester refused the requests. "I just told the guys, 'You can't get in the airplane'," he said. "They just kept standing around."
Specifically what the men planned to do with the crop-dusters is unclear but there are fears that they intended to use them to spray deadly germs or chemicals.
A government official said: "The theory is that they were looking into this as a back-up to their main objective or else as a whole other type of operation that could still be a concern. There are certainly enough questions to elevate our concerns."
Intelligence sources have already said they believe that Osama bin Laden and his network may have access to such weapons.
Donald Rumsfeld, the US Defence Secretary, said at the weekend that countries sponsoring terrorism had "very active chemical and biological warfare programmes." He added: "We know that they are in close contact with terrorist networks around the world."