WASHINGTON -- A jittery Senate faced its second attack with a deadly toxin in 28 months on Tuesday, this time in the form of ricin powder sent to Senate Majority Leader Bill Frist. Another letter containing ricin and bound for the White House had been intercepted in November, a law enforcement official disclosed.
No illnesses were reported in either case, but dozens of Senate workers were being monitored and work in the Senate slowed to a crawl.
Frist and others said tests overnight showed the substance was ricin, a natural and potent poison made by refining castor beans. Frist said the ricin was active, or capable of causing illness, but tests measuring its potency were incomplete.
Health officials urged Senate staff to watch for swiftly developing fever, coughs or fluid in the lungs over the next two or three days. When inhaled in sufficient quantities or injected, ricin can be fatal -- and there is no known cure.
Health experts expressed optimism that casualties would be averted in the new attack. None of the dozens of congressional employees who were near the Tennessee Republican's office on Monday when the white powder was discovered was believed to be sick.
"As each minute ticks by, we are less and less concerned about the health effects," said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.
The ricin-laced letter addressed to the White House had been detected at an offsite mail processing facility, the law enforcement official said, speaking on condition of anonymity.
The investigation into that letter continues, and there have been no arrests, the official said. Authorities determined the letter posed no threat to health because of the ricin's low potency and granular form.
On Capitol Hill, all three Senate office buildings were shut Tuesday and were to be closed today too. They could be closed the rest of the week.
That included the Dirksen Senate Office Building, where the substance was found Monday afternoon by a young worker in Frist's fourth-floor mailroom. A sign stating "Closed" hung from one of Dirksen's main doors. Yellow sheets cordoned off areas inside.
The Capitol building -- where heavy security and a persistent case of nerves have reigned since the terrorist attacks of Sept. 11, 2001 -- was closed to tourists.
The FBI and other agencies were conducting tests on the substance. At Fort Detrick, Md., Army scientists were using electron microscopes to determine the size of the ricin's particles -- crucial to determining whether any of it may have been inhaled.
Frist said 16 potentially exposed staff workers had been quarantined Monday night and decontaminated with showers. Spokesman Bob Stevenson later raised that figure to 24, plus an uncertain number of Capitol police officers who took precautionary showers after their shifts.
But other Senate aides, including at least one who was quarantined, said the figure was 40 to 50, including about 10 Capitol police officers and aides to Frist, Sen. James Jeffords, I-Vt., and the Senate Environment and Public Works Committee.
Across the Capitol, the House conducted business as usual. Senate leaders decided to hold no votes and canceled all committee hearings, though senators trooped to the chamber floor to debate a highway bill.