Between 40 and 50 Capitol employees were quarantined briefly and decontaminated, said Senate aides who spoke on condition of anonymity.

But officials have found no evidence that anyone was significantly exposed to the poison enough "to make them sick," said Dr. John Eisold, the Capitol physician. However, he urged employees to be alert for symptoms over the next 48 to 72 hours.

Frist said tests confirmed that the powder was ricin: "It is active, how active we don't know," meaning that it could potentially sicken people. But he said he was confident that everyone who was at risk has been identified.

U.S. Capitol police chief Terrance Gainer said everything that was in the 4th floor mailroom in Frist's Dirksen Senate Office Building office has been seized, but that authorities have not yet analyzed all of the mail.

Sen. Tom Daschle, the Democratic leader, likened the events to the 2001 incident in which his office received letters containing potentially deadly anthrax.

"Terrorists acts, criminal attacks of this kind, will not stop the work of the Senate or the Congress," Daschle said at a news briefing.

Eisold said there were no apparent cases of poisoning among those working in the affected building, but said that health officials are following closely any employees who report flu-like symptoms to be sure.

"We remain vigilant," Daschle said. "People should err on the caution side. If there is a question they should see us."

The discovery forced the Senate to cancel much of its business Tuesday, although the chamber's leaders initially made a show of going forward. Senate office buildings where 6,200 people work were closed and the much of the Capitol Hill area were eerily quiet. Sen. Robert Bennett, R-Utah, said it would be four or five days before the buildings would be reopened.

Police told lawmakers not to open mail. As a precaution, the Postal Service closed its facility that handles government mail. Gainer said investigators were "working through the Capitol complex" to make sure there is no ricin anywhere else.

Mark Saunders, spokesman for the U.S. Postal Service, said, "Out of an abundance of caution," officials late Monday closed the facility that handles congressional mail after preliminary tests showed a suspicious powdery substance.

Frist told senators at their weekly luncheon Tuesday that the powder apparently came from a stack of 40 letters being opened by a machine, Sen. John Cornyn, R-Texas, told reporters.

All three Senate office buildings were closed to permit inspection even though the powder found Monday was only in the Dirksen building.

But federal health officials said it was good news that none of them had become ill.

"As each minute ticks by, we are less and less concerned about the health effects," said CDC Director Dr. Julie Gerberding. If the ricin were pure, she said, "We would expect very early onset. The fact that we haven't seen that is reassuring."

President Bush (news - web sites) was briefed on the situation, and the administration established an interagency team to investigate what Frist told colleagues was a chilling crime.

The tense atmosphere brought back for lawmakers and staff the realization of life in the era of terrorism; the Capitol has effectively has had its guard up since the terror attacks of Sept. 11, 2001.

In this instance, police told lawmakers not to open any mail. Mail to congressional offices has been irradiated since the 2001 anthrax attack, but radiation would not have an effect on ricin, Frist said.

A simple "Closed" sign was tacked onto one of the main, ornate doors of the Dirksen Senate Office Building that housed Frist's office. Through a window of that building a pile of red, plastic bags could be seen in the hallway. Yellow sheets were erected to cordon off areas off the hall.

Frist said he had been told "the definitive test" on the powder "said it was ricin for sure." Frist said he was referring to a type of testing known as PCR or polymerase chain reaction, that detects a gene from the castor plant from which ricin is made.

The CDC planned additional PCR tests to confirm if the powder contained ricin. Also, Army scientists were to inject samples of the powder found in Frist's office into laboratory animals to see if they become ill, said a federal health official, speaking on condition of anonymity.

A federal law enforcement official, speaking on condition of anonymity, said no threatening letter or note linked to the powder has been found.

Frist told colleagues at the opening of the Senate session, "Somebody in all likelihood manufactured this with intent to harm."

Some senators opened temporary work areas in the Capitol.

"There's sort of an odd sense of deja vu with the anthrax and that this is happening again," said Tessa Hafen, a spokeswoman for Sen. .Harry Reid of Nevada, the senate's No. 2 Democrat.

In 2001, an anthrax-laced letter shut down Congress briefly and closed the Hart Senate Office Building for months of expensive cleaning. Five people were killed and 17 sickened nationwide after coming into contact with letters containing anthrax. An investigation continues.

A clue to ricin poisoning is a suddenly developed fever, cough and excess fluid in the lungs, a fact sheet from CDC says. These symptoms could be followed by severe breathing problems and possibly death, it said. There is no known antidote.

___

On the Net:

Senate site: http://www.senate.gov

Centers for Disease Control and Prevention (news - web sites) facts about ricin: http://www.bt.cdc.gov/agent/ricin/facts.asp