Ricin was sent to the White House in November, it was disclosed today – two days after the deadly poison was found in a Senate post room.
Several Capitol buildings remained closed as experts combed the offices and hallways for further traces of the toxin.
Law enforcement sources confirmed that ricin was posted to the White House in November – but the incident was not made public and the presidential bodyguard, the Secret Service, delayed sharing the information with the FBI.
A month before the ricin was sent to the White House, a similar package was intercepted at a postal depot which serves a South Carolina airport.
Letters in both packages were signed from “Fallen Angel” and made demands for the rights of truck drivers, suggesting domestic, rather than international, terrorism.
Fallen Angel threatened to poison water supplies.
The discovery of the ricin in the office of Republican Senate Majority Leader Bill Frist, on Monday night, sparked memories of the anthrax attacks of 2001.
It is understood that the powder was found by an intern on a post sorting machine in the office, but the exact package from which it originated has not yet been identified.
Tests showed it was potent enough to kill in small quantities.
After the find, employees were hosed down in the hallway of the Dirksen Senate Office Building.
Some wore laminated cards around their necks, with the names and telephone numbers of their immediate colleagues.
The Dirksen building, as well as the Hart and Russell Senate office buildings, will remain closed until the weekend, according to the Senate website.
Senators would continue to meet, although much of their work would inevitably be disrupted by the closure of the offices.
The discovery was being treated as “terrorist activity“, and security around the Capitol has been tightened.
A chemical and biological weapons response unit from the US Marines was among those called in to support police and the FBI.
Fears of a second ricin discovery at a Connecticut postal depot proved unfounded.
Senators remained defiant and convened in Washington as scheduled, although tourists were banned from the area “until further notice“, the Senate web site added.
Postal deliveries to the House of Representatives were also suspended, according to a memo from the House leadership.
Officials and politicians were warned not to open letters or packages.
The ricin – a toxin twice as deadly as Cobra venom – was found at 8pm Irish time on Monday.
The poison, derived from the castor bean plant, is extremely deadly in tiny amounts.
Calling it a “criminal action” and “terrorist activity“, Mr Frist has said the powder was “sent with intent to harm”.
Despite the fears, no-one has reported injuries or illness.
There were no indications that the ricin had spread through the building’s ventilation systems.
Ricin poisoning causes a sudden fever, cough and excess fluid in the lungs, followed by severe breathing problems and possibly death. There is no known antidote.
In 2001, an anthrax scare gripped the United States. Five people were killed in the anthrax attacks.
The anthrax cases still have not been solved.