Trace amounts of anthrax were found in the mail room in the Veterans' Affairs Medical Centre in Washington yesterday when the authorities were due to order the release of powerful chemicals in a Senate office building to try to kill any lingering anthrax spores.
Government officials will today question a man detained in a town at the centre of the American anthrax outbreak.
FBI agents raided at least two apartments in Trenton, New Jersey, where three of the letters contaminated with the potentially deadly spores were sent.
Witnesses said agents took one man into custody after carrying out a three-hour search in an apartment where four "Middle Eastern" men were living. The man was understood to have been detained by immigration authorities. Two other people were reported to have been detained near Trenton post office, which has emerged as the only known source of the anthrax contamination.
The FBI said there was no direct link between the 11 September terror attacks, the anthrax poisoning and the raids on the apartments.
One month after the first anthrax case was confirmed, President George Bush on Saturday called the anthrax threat "a second wave of terrorist attacks upon our country". He said in his weekly radio address that the government was working swiftly to test post offices and other sites for spores.
The Centres for Disease Control and Prevention (CDCP) confirmed the presence of anthrax at the Veterans' Affairs Medical Centre, said a spokesman for the centre, Phil Budahn. Five mail room workers have been taking antibiotics since 25 October as a precautionary measure.
Mr Budahn said the centre's 250 patients would be monitored but it was extremely unlikely the anthrax had spread beyond the mail room, which closed on Wednesday for cleaning. "No one is ill," he said. "There's no indications that patients or other staff came in contact with hazardous material. This is purely a mail room issue."
The centre received mail from Brentwood, a Washington postal centre that processed an anthrax-laced envelope delivered to the Senate majority leader Tom Daschle's office.
On Capitol Hill, environmental experts were due to announce plans for decontaminating the Hart Senate Office Building, where the letter to Mr Daschle was opened.
They plan to fill the nine-storey building with bacteria- killing chlorine dioxide gas but the final approval was being left to a panel of experts.
To date, the biological attack has killed four people and infected 13 others. Although concentrated along the east coast, anthrax has also been found in Kansas City and Indianapolis.
The New York Times reported that the CDCP had vaccinated 140 members of epidemiology teams that can be dispatched on short notice to examine a suspected case of smallpox anywhere in the country. Unlike anthrax, smallpox is easily spread from person to person and federal officials are rushing to stockpile enough vaccine to inoculate millions of Americans if necessary.