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.
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 on Tuesday and 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.
Frist and others said tests overnight showed the substance was ricin, a natural and potent poison made by refining castor beans.
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 vaccine or cure.
Frist's offices in Tennessee were also closed as investigators checked mail there, said Frist spokesman Nick Smith.
In Washington, senators gave many aides the day off and brought others to work in small Capitol offices the lawmakers normally use as private hideaways.
The FBI and other agencies were conducting other tests. 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.
Senate leaders made a show of calm and control. They said they had refined their ability to respond to emergencies since the anthrax attacks of late 2001 with better communications and coordination.
"Things are going very well, not perfectly, but very, well," said Frist, a medical doctor who has advised Capitol colleagues about potential terror attacks through the mail ever since the anthrax letters of late 2001.
Frist said 16 potentially exposed staff workers had been quarantined Monday night and decontaminated with showers. But 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.
At a briefing for reporters, Frist said there was not yet information on how dangerous this sample of ricin powder was. Democratic leader Tom Daschle of South Dakota said tests of air filters showed the chemical had not been circulated through the buildings' ventilation systems.
But Sen. Lindsey Graham, R-S.C., emerging from a lunch where Frist, Capitol police chief Terrance Gainer and Capitol physician John Eisold briefed Republican senators, said the three had expressed concern.
"There was something specific about this that made them worry," Graham said. "Somebody knew what they were doing. ... Frist said the type, the way it was presented indicated that people understood it goes into the air and gets into lungs."
There were also questions raised about how effectively senators and aides were told about the attack and the potential jeopardy they faced.
"We weren't notified promptly enough yesterday," said Sen. Tom Harkin, D-Iowa, who said one of his aides worked well into the evening in the Dirksen building. "But that's OK, people make mistakes."
One aide who was quarantined — which did not occur until 6:30 Monday evening — said many coworkers had already gone home. This aide said those quarantined were asked to telephone colleagues who had left and tell them to shower and put their clothes in a bag.
Frist and police chief Gainer said investigators were still uncertain which, if any, piece of mail the ricin had come from. Gainer said officials had not yet found any "visible threat," such as a menacing letter. The ricin was found on a device that opens mail, authorities said.
Workers began retrieving mail from all Senate and House offices as authorities worried that contaminated mail may have been sent to other lawmakers. In October 2001, anthrax-tainted letters were mailed to then-Senate Majority Leader Daschle and to Sen. Patrick Leahy, D-Vt.
Officials said there was no evidence of ricin elsewhere in the Capitol complex, though as a precaution the postal facility that processes Congress' mail was shut. In 2001, two postal workers in Washington were among five people who died from anthrax exposure.
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.
"Terrorist attacks and criminal acts of this kind won't stop the work of the Senate or the Congress as we have important work to be done," said Daschle.
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