The failure to solve one of the most baffling and sinister terrorist cases of modern times has not only led to intense frustration for the FBI, but has also prompted the British widow of one of the victims to sue the US Government.

Bob Stevens, a British picture editor from Berkshire who worked in Boca Raton, Florida, was one of five people who died in and seventeen who became ill in September and October 2001, after coming into contact with a weapons-grade strain of anthrax posted to media organisations and the offices of two Democratic senators in Washington.

His widow, Maureen, who believes that the anthrax came from a government biodefence laboratory in Maryland, spoke yesterday of her anger and frustration at the failure of the FBI to make an arrest in the case. She is suing the Government for $30 million (nearly £17 million), alleging that security lapses at the US Army Medical Research Institute for Infectious Diseases in Fort Detrick led to her husband’s death. Much of her case is aimed at getting leading bioterrorism experts to testify in court.

“There’s nothing coming out — it’s just amazing,” Mrs Stevens said. “I’ve had one meeting with the FBI. I have had little communication with them. I would have thought they wanted to talk more.”

She said that she found the case difficult to talk about, because of anger and other emotions, and that she and her lawyer were “just meeting a stone wall”.

The failure to make one arrest in the case has astounded and dismayed many Americans. FBI agents and officials from the Postal Inspection Service have conducted more than 8,000 interviews on four continents and served more than 5,000 subpoenas. They have travelled to Afghanistan twice.

In the past year, the FBI says, the number of agents on the case has dropped from 31 to 21, a far cry from the hundreds assigned to the investigation in its early weeks. Despite a $2.5 million reward for information leading to a conviction, the case “is going nowhere”, a former investigator said. The favoured theory has remained consistent: that the culprit is an American scientist who had access to the anthrax.

Spore-laden letters were posted on September 18 and October 9, 2001, to media organisations in New York and Florida, and to the offices of Tom Daschle, then the Senate Democratic leader, and a colleague, Senator Patrick Leahy, of Vermont. Five people were killed — Mr Stevens and two postal workers, and also a New York hospital worker and an elderly Connecticut woman, whose deaths were not a direct result of the mail attacks.

Panic gripped Washington, with the Senate, House of Representatives, Supreme Court building and numerous postal facilities being shut down. The letters included photocopied notes referring to the September 11 attacks and Islamic rhetoric.

Investigators soon determined that the anthrax used was the Ames strain, most commonly used in American biodefence research. Attention focused on Fort Detrick, and in particular on Steven Hatfill, an American biodefence expert who worked at the facility between 1997 and 1999. Dr Hatfill, who has not been charged and fiercely denies any involvement, was named as a “person of interest” by John Ashcroft, then the Attorney-General.

Numerous tips have proved fruitless. Two years ago the FBI spent three weeks draining a pond near Fort Detrick, believing that the culprit may have discarded materials there. The pond yielded nothing.

Another tip, apparently from an inmate in Guantanamo Bay, led agents to fly to Kabul, the Afghan capital, in May last year, and then to the Kandahar mountains — but nothing was found. US scientists have still not been able to identify the laboratory from which the anthrax came, but other facilities have been investigated, including one at Louisiana State University and another in Utah.

DEATH IN THE POST

Sept 25, 2001 Erin O’Connor, an assistant to an NBC journalist, opens a letter postmarked Trenton, New Jersey, containing a brown granular substance

Oct 5 Bob Stevens, picture editor of the Sun newspaper, dies in Boca Raton, Florida. Traces of anthrax are found on his computer keyboard

Oct 10 Anthrax is found in a sorting office that handles post for the White House

Oct 12 A case of anthrax is reported in New York City; O’Connor becomes the first person to test positive for skin anthrax

Oct 15 A letter containing anthrax is received by Tom Daschle, the leader of the United States Senate

Oct 21 and 22 Two male postal workers at the office in Washington that sorts post for Capitol Hill die

Oct 29 Fresh anthrax spores are detected at the Supreme Court, prompting justices to meet outside the building for the first time in its 66-year history

Times Recommends

• No cause for celebration in crushed Sarajevo
• Jazz funeral for the final victims of Katrina
• French fined for 'insulting' civil servants