A letter contaminated with anthrax, according to two preliminary field tests, has been received by the US Senate Majority leader, Mr Tom Daschle.
The Democrats' South Dakota leader was not near the envelope when it was opened yesterday morning, but staff in his office in the Senate Hart building, beside the Capitol, are being tested and treated with antibiotics.
The sixth floor of the building was sealed off as police removed the heavily taped letter containing a white powder which tested positive. The letter, which arrived on Friday and was only opened yesterday, was postmarked Trenton, New Jersey, the same as that received by NBC.
The FBI warned the preliminary tests were not very reliable and had been producing false positives. But new procedures for vetting mail have been introduced in Congress with the complete recall of all post delivered yesterday.
Speaking to reporters at the White House, President George Bush, who announced the incident, said "there may be some possible link" between Osama bin Laden and the recent flurry of anthrax-related developments.
"I wouldn't put it past him but we don't have any hard evidence," he said.
Speaking to journalists on the steps of the Capitol later, Mr Daschle said he was angry and disappointed at the attack but he was determined to ensure Congress remained open and accessible. He echoed the President's call to the people not to let such incidents change their lives.
"We have to lead our lives," he said. "We intend to."
The scare, the first in the nation's capital, again reinforces the growing conviction the attacks are politically motivated but the US security authorities yesterday also insisted there was no evidence to connect the anthrax attacks in Florida, New York, Nevada and now Washington to the al Qaeda attacks on September 11th.
To date, the only connection established, according to the FBI appears to be a bizarre coincidence: Ms Gloria Irish, the wife of the editor of the tabloid the Sun, Mr Michael Irish, is an estate agent in Delray Beach, Florida. This summer, two of her clients were Hamza Alghamdi and Marwan al-Shehhi, both of whom were to die as hijackers on September 11th. Her husband's office is the centre of an anthrax scare.
Confirmed evidence of anthrax attacks yesterday otherwise remain confined to those incidents already reported - that on American Media Inc. in Baco Raton, Florida; on NBC, in New York; and on the Microsoft office in Reno, Nevada - although late on Sunday the mayor of New York, Mr Rudy Giuliani, announced one police officer and two lab technicians also showed signs of exposure although no infection.
The three, all wearing masks, were involved in collecting the NBC letter.
The authorities will now tighten up on handling procedures. "There's a very strict lab protocol in place, but what may have happened here, it's possible they may not have fit their masks properly around their noses and mouths," said Ms Andra Mullin, a health department spokeswoman. She said rules are being changed to require technicians testing suspicious substances to wear protective hoods as well as masks.
In Reno, where a letter sent to a Microsoft office contained pornographic pictures contaminated with anthrax, officials said tests of four people who may have been exposed proved negative for the bacteria. The total numbers so far exposed nationally is 12, with only two infected and one death (Robert Stephens in Floida). A possible case in New Jersey remains unconfirmed.
Hoaxes have continued to complicate investigations with the Attorney General, Mr John Ashcroft, warning that those involved will be prosecuted. Police figures show a dramatic increase in biological threats since 1998 when biological warfare received considerable media coverage.
Last night, a US abortion rights group said 90 of its clinics and offices in at least 13 states had received envelopes containing threatening letters and an unidentified powdery substance. The group said an initial field test on the substance in a letter sent to offices in Greensboro, North Carolina, had come back as negative for anthrax. Officials are conducting further tests.
The US authorities have announced a major expansion in their programme to deal with the biological warfare threat. The Secretary of State for Health and Human Resources, Mr Tomy Thompson, has asked Congress for an additional budget of $1.5 billion and has ordered a six-fold increase in antibiotics stocks.