Supreme Court justices are taking antibiotics
A diplomatic pouch sent to Lima, Peru, from the State Department is also contaminated, initial tests show.
The new discoveries in Washington add to a growing list of federal facilities disrupted by the anthrax scare.
So far three people have died from the outbreak - two of them postal workers.
The Supreme Court closed on Friday after anthrax was found in an off-site mail warehouse, and justices convened elsewhere for the first time in the court's 66-year history.
But on Monday it was confirmed that the bacteria had been found in the court's mail room.
Staff given antibiotics
Traces of anthrax have been found at two locations inside the State Department - a mail pouch sent to the US Embassy in Peru and in mail sent to another State Department building in Washington, department spokesman Richard Boucher said on Monday.
He added: "The ventilation system sampling in this building has come out negative. So as far as we know nothing has spread beyond mail rooms."
All staff who handle mail are receiving antibiotics, he said.
The pouch had been sent from the department's
off-site mail facility in Sterling, Virginia, where an employee last
week contracted inhalation anthrax.
Health and Human Services Secretary Tommy Thompson said that anthrax traces had been detected at the Cohen Building in Washington DC which houses his own department and the headquarters of the Voice of America radio station.
Traces of anthrax were also found at an off-site mail facility for the Justice Department, officials said on Sunday.
New machines
The facility in the Washington suburb of Landover, Maryland, also handles mail for the office of Attorney General John Ashcroft.
Mr Ashcroft is leading the hunt for those who sent the anthrax-laced mail.
Anthrax has also been found at mail offices that serve the CIA, Congress, the White House and other government and military offices.
Sites in Florida, New York and New Jersey have also been contaminated.
The postal service is spending $40m on eight electron-beam devices to sanitise letters and packages. The equipment will be used first in Washington DC.
As America's highest institutions continued to be disrupted by the attacks, health officials confirmed the latest infection.
A female postal worker in New Jersey has inhalation anthrax, the most deadly form of the disease.
She worked as a mail handler at the Hamilton sorting office which handled the contaminated letter sent to the Senator Tom Daschle, NBC News anchorman Tom Brokaw and the New York Post newspaper.
Her condition is said to be improving, five days after she first came under observation for suspected anthrax.
The three dead were among eight people who have contracted inhalation anthrax.
Information more frequent
Six others have been diagnosed with skin anthrax and thousands more have been given antibiotics for the disease.
The White House says there could still be undiscovered letters containing anthrax in the American postal system and chief of staff Andrew Card warned people to take extra care.
To keep the American public up to date with developments in the anthrax threat, homeland security chief Tom Ridge will begin briefing the media at least three times a week, administration officials said.
Dr Ivan Walks, Washington's public health director, and Dr Anthony Fauci, of the National Institutes of Health, said the antibiotic doxycycline was now being recommended in place of Ciprofloxacin because it had fewer side effects.
FBI and CIA officials are now reported to believe the anthrax contamination is the work of extremists in the United States rather than terrorists abroad.