At the same time, health ministers of the G7 countries and Mexico met in London and agreed to carry out an international exercise to test reactions to a biological, chemical or radio-nuclear terrorist incident.

Barbara Rosenberg, the director of the Federation of American Scientists' Chemical and Biological Weapons Program, raised the possibility that the CIA could have ordered a "field trial" on the possible effects of delivering anthrax through the mail and the contents could have been used by whoever was responsible for the anthrax attacks.

Dr Rosenberg told the BBC's Newsnight: "Some very expert field person would have been given this job and it would have been left to him to decide exactly how to carry it out.

"The result might have been a project gone badly awry if he decided to use it for his own purposes and target the media and the Senate for his own motives as not intended by the government project."

Dr Rosenberg claimed the culprit had knowledge both of the law and of the detective work it would need for him to be caught. She said: "This person knows a lot about forensic matters, knows exactly what he can be prosecuted for and what he can get away with and I think he had some personal matters that he might have wanted to settle, but I think in addition that he felt that bio-defence was being underemphasised for some time in the past."