At 10 o’clock on a warm autumn morning in Washington, a secretary was opening mail in the office of Senator Tom Daschle, the Senate majority leader. She slit open a hand-lettered envelope that had the return address of the fourth-grade class of a New Jersey school. She removed a sheet of paper and powder fell out, the colour of bleached bone, landing on the carpet. A puff of dust rose like smoke from a snuffed-out candle then vanished.

It was October 15, 2001. Just over a month after the terror attacks of September 11, anthrax was the new menace. Letters containing greyish, crumbly, granular anthrax had arrived at media offices in New York. Several people had contracted cutaneous anthrax. A man had died in Florida from inhalation anthrax.

The secretary, whose name has never been revealed, took no chances: she threw the letter into a wastepaper basket and called the police.

A team of agents from the hazardous materials response unit (HMRU) of the Federal Bureau of Investigation hurried from their headquarters in Quantico, Virginia. They put on protective suits with masks and respirators, retrieved the letter from the bin and did a rapid test for anthrax, stirring a bit of the powder into a test tube. It came up positive.

They wrapped the envelope and letter in sheets of aluminium foil, put them in Ziploc bags and cut out a piece of the carpet. They put all the evidence into plastic containers sealed with red evidence tape.

Two special agents put the containers in the boot of an unmarked FBI car and drove north to Fort Detrick, an army base in Frederick, Maryland. It was the centre of the American army’s germ weapons programme until 1969, when President Richard Nixon shut it down.

The agents parked by the United States Army Medical Research Institute of Infectious Diseases (USAMRIID), America’s principal biodefence laboratory. Its mission is to develop defences against biological weapons and to help protect the population against a terrorist bioattack.

The main building of USAMRIID is a windowless monolith that covers seven acres. Near its centre are biocontainment suites: groups of laboratory rooms sealed off and kept under negative air pressure so nothing contagious will leak out. The suites are classified at differing levels of biosecurity, from biosafety level 2 to level 4, where scientists in “spacesuits” work with lethal, incurable viruses.

The agents handed the sealed containers to John Ezzell, a bearded anthrax specialist. Over the years he had analysed hundreds of samples of “anthrax” collected by the HMRU. The samples had all proved to be hoaxes or incompetent attempts to make anthrax — baby powder, dirt, you name it.

In green surgical scrubs, gloves and booties, and wearing a respirator, he carried the containers into a warren of labs in suite AA3 and placed them inside a laminar-flow hood: a glass safety cabinet with an open front in which the air is pulled up and away from a sample, preventing contamination.

Ezzell opened the containers and carefully unwrapped the aluminium foil. A smooth, pale tan powder started floating into the air and up into the hood. The envelope contained about two grams of pale, uniform, light tan powder. There was also a message in block capitals:

09-11-01
YOU CAN NOT STOP US.
WE HAVE THIS ANTHRAX.
YOU DIE NOW.
ARE YOU AFRAID?
DEATH TO AMERICA.
DEATH TO ISRAEL.
ALLAH IS GREAT.

Using a spatula, he slowly lifted out a small amount of the powder. He wanted to get it into a test tube, but it started dancing up and away into the hood. A key element in the design of a military bioweapon is its intrinsic energy — the capacity of the particles to fly into the air and form an undetectable cloud, which can travel long distances and fill a building like a gas.