Some people say it's difficult to make anthrax - in my opinion if you have basic microbiology and biotechnology knowledge, it's not that difficult

Dr Kenneth Alibek

It is not inconceivable that groups with extensive contacts and sufficient funding could gain access to both of these.

Former Soviet bioweapons scientist Dr Kenneth Alibek, who has helped grow anthrax, told the BBC: "Some people say it's difficult to make anthrax - in my opinion if you have basic microbiology and biotechnology knowledge, it's not that difficult."

However, once made, anthrax spores can be stored for considerable periods of time.

To be the most effective, the resulting spores would have to be distributed in aerosol form - so that they can be spread in the air over wider areas and be inhaled by victims, causing the most feared respiratory form of anthrax.

It is this delivery system which is most problematic to arrange.

The reports linking World Trade Center attackers with enquiries about crop-dusting planes certainly suggest they might have been looking for a delivery system, but experts say that these planes would have been most unsuited to the task.

How deadly is it?

Once anthrax spores have lodged in the lung and caused an infection, nine out of 10 patients die.

If antibiotics are given early, this will prevent only one of those nine deaths.

The deadliness of an anthrax attack depends obviously on the quantity of spores and the effectiveness of a delivery system.

Because the disease is not contagious, only those directly exposed to the spores have any chance of falling ill.

In 1970, the World Health Organisation estimated that, should 50kg of anthrax be released from an aircraft over an urban population of five million, there would be 250,000 cases of the disease.

A 1993 report estimated that releasing a cloud of 100kg of spores upwind of Washington DC could cause between 130,000 and 3m deaths.

However, the production of these quantities of anthrax is a significant undertaking.

Following an aerosol attack, there is a period - dependent on prevailing weather conditions - in which spores remain airborne.

Once they do settle on the ground, it is unclear whether they can cause infection if they are stirred up and re-suspended in the air.

However, evidence from the Soviet outbreak, in which no cases developed long after the release of anthrax, despite no real efforts to decontaminate, is seen as reassuring by scientists.

Certainly, any effort to completely decontaminate large areas of ground would be considered virtually impossible by many.

When the UK military tested spore delivery systems on the tiny Gruinard Island off the Scottish coast during World War II, spores persisted and remained theoretically capable of infection for decades afterwards.

A massive decontamination effort, started in 1979 and completed in 1987, used 280 tons of formaldehyde and 2000 tons of seawater.