Investigators are still looking into whether the discovery of the poison ricin in a U.S. Senate office building mailroom Monday is linked to a similar incident at the White House last November.
White House spokesman Scott McClellan said Wednesday that he does not believe President Bush was notified last year when a letter containing a suspicious substance was sent to the White House.
Law enforcement sources confirmed Tuesday that the substance was ricin, a potentially fatal poison derived from castor beans, and for which there is no antidote.
But the sources said the ricin contained in the letter had a low potency and posed no health risk at the time.
Meanwhile, Senate Majority Leader Bill Frist says three Senate buildings that were closed after the ricin was found in his mailroom will re-open, one by one, on Thursday, Friday and next Monday.
Senator Frist says no one exposed to the substance found in the mailroom has become ill.
In 2001, deadly anthrax spores were discovered in another Senate office building. While no Senate staffers were affected, two postal workers at a nearby Washington D.C. post office died.