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.
Police 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 Mr 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.
PA