Only five laboratories, including the defence science and technology laboratories at Porton Down, Salisbury, have been found to have spores of anthrax identical to the bacteria sent through the post to two Democratic senators and news organisations in New York and Florida.
But frustrated FBI agents say they have not been able to find enough information about security at Porton Down - one of the most secretive establishments in Britain - to decide whether it could be the source of the terrorists' anthrax.
Another focus of the FBI inquiry is the CIA, which has been conducting experiments on anthrax in the interests of defence from germ warfare.
Both Porton Down, directly, and the CIA, indirectly, received their samples of the particular anthrax spores used in the attacks from the US army medical research institute of infectious diseases at Fort Detrick, about 50 miles north of Washington.
Sources in the FBI said the CIA was under investigation because of the bureau's "interest" in a contractor which used to work for the agency in its anthrax project.