The Supreme Court shut down for tests
It was not clear how the anthrax, which was not in sufficient concentrations to be a significant health risk, came to be in the offices.
Also on Friday, the US Supreme Court closed down after anthrax was found on a filter at its mail-sorting office.
Health officials recommended antibiotics for thousands more people to prevent possible new infections.
Domestic or foreign?
On Friday, the White House said that anthrax found in a letter sent to Senate majority leader Tom Daschle may have been produced within the United States.
Spokesman Ari Fleischer said scientific advisers had concluded that "a PhD microbiologist in a sophisticated laboratory" could have been behind the outbreak.
On Friday, ABC television quoted three anonymous sources as saying that intial tests on the letter sent to Mr Daschle showed a chemical that is typical of Iraq's biological weapons' programme.
Bentonite makes anthrax more deadly by keeping the particles separate and thus more easily airborne and more easily inhaled.
However, Mr Fleischer denied that tests on Mr Daschle's letter had revealed traces of bentonite.
More tests
The three congressmen who offices showed traces of anthrax were John E Baldacci, Rush Holt and Mike Pence who all have offices on the sixth and seventh floors of Longworth House at the Capitol.
Anthrax was discovered in two more locations on Friday: a Central Intelligence Agency mail handling building at its headquarters in suburban
Virginia, and on a filter
removed from a warehouse that screens mail for the US Supreme
Court in Washington.
Federal officials said there was no evidence that spores were in the court building itself.
They are the latest in a series of cases linked to the mail, which have prompted the US Postal Service to carry out checks on government-related mailrooms, hand out antibiotics to thousands of workers and purchase irradiation equipment to treat letters.
Two postal service workers, who were employed at the Washington sorting office that handled the letter sent to Senator Daschle died, earlier this week after contracting inhalation anthrax. Two of their colleagues are receiving treatment for the same form of the disease.
At present the only known source of anthrax in Washington is the letter sent to Senator Daschle. Investigators say either there are more undiscovered letters or anthrax spores are being spread through the postal system after being picked up by sorting machines.
In New York, anthrax has been detected on four mail-sorting machines at a processing station that handles millions of parcels a day.
Florida outbreak
In an earlier outbreak at a tabloid newspaper in Florida, two men contracted inhalation anthrax, one of whom died.
There have also been cases of cutaneous, or skin anthrax, a less serious form of the disease, in several New York media offices and mail offices in New Jersey. Seven people are known to have been infected with it.
So far, investigators have failed to trace the source of the anthrax or pinpoint a possible culprit, but identifying the method used to treat the spores may narrow the field.
Officials have no evidence of a link to those who planned the 11 September terror attacks on New York and Washington, but President George W Bush says he "wouldn't put it past" Saudi-born militant Osama Bin Laden, believed to have masterminded the attacks.