Senate Majority Leader Bill Frist has confirmed that a white, powdery substance discovered yesterday in mail delivered to his office has tested positive for ricin poison. Three Senate office buildings have been closed as a result, although the Senate will still be in session today.

A Connecticut postal worker discovered a similar substance in an envelope addressed to the Republican National Committee. No test results from this second discovery have been released publicly at this point.

Already, parallels are being drawn between this ricin attack and the 2001 anthrax attacks. The anthrax attacks targeted Capitol Hill officials and various media outlets in New York and Florida.

Ricin, which is derived from castor beans, is one of the most potent natural poisons. It can kill after being eaten, inhaled, or injected even in small amounts. There is no antidote.

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