IN SEPTEMBER 2001, just one week after 9/11, someone placed anthrax spores into seven envelopes and mailed them to five media outlets and two US senators. As a result, 22 people caught anthrax and five of them died.
The Federal Bureau of Investigation (FBI) immediately began an inquiry, codenamed Amerithrax – the largest microbial forensic investigation ever, and only now are the experts involved revealing their findings.
Anthrax is an ancient and much feared disease caused by the spore-forming bacterium Bacillus anthracis.
It used to cause huge epidemics among domestic animals, and as spores are hardy in the environment, this is the usual source of human disease.
Infection may either cause a skin infection, known as a "malignant pustule", which generally affects animal handlers, or, more seriously, inhalation anthrax.
In the latter condition, the spores germinate in the lungs before spreading to other vital organs producing lethal toxins as they go.
These days, inhalation anthrax is extremely rare, with only 18 cases reported in the US in the whole of the 20th century.
Untreated, the death rate approaches 100 per cent, but the disease responds to antibiotics if caught early enough.
In the 2001 attack there were 11 cases of skin, and 11 cases of inhalation anthrax, with all five deaths among those who inhaled the spores.
Biological weapons are still under production in at least 13 countries, and anthrax spores are high on the desirable list for bio-warfare.
They are the ideal lethal agent – light (100 billion spores per gram), odourless, invisible, stable, easily aerosolised and lethal if inhaled. With just one to three spores capable of causing infection, the seven envelopes sent in 2001, containing around 2 grams each, could have caused a massive epidemic.
In the aftermath of 9/11, rumours about the perpetrator of the anthrax attack abounded, with al-Qaeda being the favourite suspect, but the FBI soon revealed that the origin was much closer to home.
Aided by US anthrax experts who analysed the DNA sequence from the dispatched spores and microbes isolated from victims, they found that all were of the same "Ames" strain of B anthracis, with radiocarbon dating showing them to have been produced within the previous two years.
The Ames strain of B anthracis was developed at the US army Medical Research Institute of Infectious Diseases (USAMRIID) at Fort Detrick in Maryland, and then sent to 15 US research labs and six labs overseas.
The FBI analysed 1,072 samples of Ames obtained from 18 labs hoping to find a sample with unique mutations that would link it to the attack sample.
At first, the DNA from all samples proved identical, but then a lab worker at USAMRIID noticed a small number of strange-looking spores in the attack sample. When these spores were isolated and their DNA sequence determined, ten mutations were identified, which distinguished it from the common Ames DNA.
When all 1,072 samples were screened for four of these mutations, eight were found to contain all four. One of these samples came from flask RMR-1029 at USAMRIID, and the other seven had been sub-cultured from it.
With this evidence, the net closed in on USAMRIID as the source of the attack, and in July 2008 the FBI warned Dr Bruce E Ivins, the scientist from the facility who was responsible for flask RMR-1029, it was about to press charges against him.
But the case never came to court because on 27 July Ivins took a drug overdose and died two days later.
Scientists involved in the investigation say their findings point the finger at the source of the anthrax spores but not the attacker, and are pushing the FBI to publish its findings. But with potential law suits in the offing from both the victims of the attack and Ivins' family, the FBI is saying nothing.
Human DNA fingerprinting is now well accepted as evidence in a court of law, but the microbial forensic evidence presented at Ivins' trial would have been a ground-breaking.
This event will now not take place, but nevertheless the FBI will have to reveal its evidence at any civil hearings, and this test-case is eagerly awaited by scientists and the public alike.