Investigators admit they have made little progress
Part of the defence headquarters have since been decontaminated and given the all-clear, while several workers are taking antibiotics as a precaution.
The traces of anthrax were found in two post office boxes - one rented by an unidentified Navy service member and another that was not assigned.
The FBI is also investigating a new case of anthrax after traces of the bacteria were found on a videotape sent from the office of NBC news anchor Tom Brokaw to the New York mayoral office.
Mayor Rudolph Giuliani said the tape, sent for testing after an assistant of Mr Brokaw had contracted anthrax, had tested positive.
"They sent it off for testing several times and it came back as positive," Mr Giuliani told reporters on Sunday, adding that no one at city hall had been found to have anthrax.
Congress offices reopening
The first cases of anthrax appeared over a month ago and new cases of contamination are reported almost daily.
But investigators have not yet identified the source of the outbreak which has hit US government buildings, media organisations and postal facilities.
The last of the three main office buildings of the US House of Representatives was to reopen on Monday.
The Longworth building was shut down after anthrax was discovered in the offices of three representatives on 26 October.
The Hart Senate office building, where anthrax was found in a letter sent to Senate Majority Leader Tom Daschle, still remains closed.
The nine-storey building will be fumigated with chlorine dioxide gas to kill any anthrax spores without harming papers, files and art work, officials told the Associated Press news agency.
Smallpox vaccinations
About 140 US health officials are being vaccinated against smallpox, amid warnings that it could be used as a biological weapon.
Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the US had to be on its guard against possible attacks using the highly contagious smallpox virus.
On Sunday, tiny amounts of anthrax were found at a mailroom in the Veterans' Affairs Medical Center in Washington, but officials said the facility's 200 patients were unlikely to be at risk.
The centre had received mail from a Washington postal sorting office that processed an anthrax-laced envelope delivered to the office of Senator Daschle.
It also emerged on Sunday that a New York woman who mysteriously died of inhalation anthrax had handled mail at the hospital where she worked.
Officials are trying to discover how Kathy Nguyen contracted the disease after initial tests at her home and the hospital proved negative.
Four people have died of anthrax, with another 13 confirmed cases.
The FBI last week admitted it had made little progress towards tracing the perpetrators of the anthrax attacks.