Anthrax fears are growing with each new case
The news came as it was confirmed that the disease caused the death of two postal workers in Washington DC.
However, a presidential spokesman confirmed that anthrax had not reached the White House itself, and President George Bush said he did not have the disease.
The postal facility is located some six miles (10 kilometres) from the White House on property shared with a naval station and an air force base.
President George W Bush said it was another sign that the United States was engaged in a war against terrorism on two fronts.
US law enforcement officials have said there is so far no evidence linking the 12 confirmed anthrax cases with foreign terrorists, although suspicion is focused on Osama Bin Laden in the wake of the 11 September attacks on New York and Washington.
Congress has now returned to work for the first time since last week, when Senator Tom Daschle's office received a letter containing anthrax.
All mail delivery to the Capitol was suspended, and Mr Daschle says that the only way to deal with the vast mound of letters and packages that has since built up may be to burn them.
Postal threat
The two dead postal workers were employed at the Brentwood post office that sorted mail for the US Congress and handled the letter containing anthrax which was sent to Mr Daschle.
The BBC's Washington correspondent says there is now concern that hundreds of postal employees have been working for some time in an anthrax-contaminated area.
Two other workers at the post office have also developed inhaled anthrax infections - the most deadly type of the disease - and are seriously ill.
A female postal worker at the New Jersey office where the contaminated letter was postmarked is also seriously ill with suspected inhaled anthrax.
New Jersey State Health Commissioner George DiFerdinando said at a press conference that the woman was in a serious but stable condition in hospital.
Unknown contamination
The anthrax at the White House mail office was found on a mail slitter, spokesman Ari Fleischer told a briefing, adding that staff at the facility were now being "swabbed and tested".
The spokesman told reporters that the source of the anthrax was still unknown.
To fight the anthrax threat nationwide, US postal authorities have launched a $1bn security operation, which includes machines to bombard mail with radiation.
The latest deaths bring the number of people killed by the spate of anthrax attacks in America to three.
The death of the postal workers has shown that an envelope containing anthrax may not need to be opened to kill.
This has caused widespread alarm as it is not known how many people may have handled the envelope along its journey from Trenton, New Jersey, to Washington.