"I know everybody is very concerned, especially here in the Senate family," Frist said at a news conference. "I'm happy to report everybody is doing fine."
Officials also confirmed to Fox News that a letter with ricin in it was sent to the White House last November. It was intercepted at an off-site mail facility and is still being investigated; the FBI is taking the lead.
That ricin was of such low potency, it never posed a public health risk.
One source told Fox News that the letter was domestic, not international.
Law enforcement officials told Fox that seven letters containing ricin were intercepted. The one sent to the White House in November was linked to another letter discovered in October and addressed to the Department of Transportation. That letter was found at a postal sorting facility in Greenville, S.C.
These two letters with ricin had similar content — typewritten demands that changes in truckers work/sleep schedules not be implemented.
The FBI is exploring whether this week's ricin incident is linked to those two letters. However, at this point they have not found that similar demand letter.
Between 40 and 50 Capitol employees were quarantined briefly and decontaminated, said Senate aides who spoke on condition of anonymity.
Federal health officials said they were reassured that none of the workers had turned up sick, since ricin is likely to affect victims soon after exposure.
Capitol physician Dr. John Eisold said officials have found no evidence of ricin inhalation or of exposure to the poison that was significant enough "to make (anyone) sick."
But he urged employees to be alert for symptoms over the next 48 to 72 hours and said doctors and officials would continue to monitor staffers' health.
"We're not cavalier," Eisold said. "In terms of individual patients, we are following them. You rule people out by following them over time."
Military sources told Fox News that the Navy Medical Research Command (search) in Silver Spring, Md., had completed three rounds of testing on the powder discovered near Frist's office and determined the substance to be ricin.
Senate office buildings remained closed Tuesday as authorities investigated the white powder found on a mail-sorting machine used by Frist's office.
U.S. Capitol Police Chief Terrance Gainer said in Tuesday's news conference with Frist that police will continue to test the surroundings for contaminants.
"I anticipate in next few days we will send hundreds of individuals in to reclaim old mail piles while we simultaneously do analysis of the areas," Gainer said. "All exams of filters or air systems continue to be negative and that is a good sign."
Frist's office Tuesday evening sent out an official note saying that the three Senate Office Buildings — Hart, Dirksen and Russell — would be closed on Wednesday.
Police told lawmakers not to open mail. As a precaution, the Postal Service closed its facility that handles government mail.
Hazardous material crews investigated a second possible suspicious substance found on the first floor of the Capitol building around noon Tuesday. But authorities soon determined the building was safe and allowed people to go back into the Capitol.
As for the ricin found in his mailroom, Frist, a physician himself, said that "somebody in all likelihood manufactured this with intent to harm."
He told senators at their weekly luncheon Tuesday that the powder apparently came from a stack of 40 letters being opened by a machine, according to Sen. John Cornyn, R-Texas.
After the ricin was found, all Senate office buildings were shut down, forcing the postponement or cancellation of all Senate hearings. The Capitol itself remained open.
"I believe it is an act of terrorism," said Senate Minority Leader Tom Daschle (search), D-S.D. "The question is who is responsible, how widespread is this and what are the implications of it. It certainly is criminal."
The FBI was investigating the matter along with Capitol police.
After preliminary tests showed the powder to be ricin, Charles Dasey, a spokesman at Fort Detrick, Md., said scientists there were doing a "confirmatory" test on the substance. The test is "higher reliability" but will take longer, he said.
The Centers for Disease Control and Prevention (search) said officials were somewhat reassured because no one had turned up sick from exposure to ricin by Tuesday afternoon.
"As each minute ticks by, we are less and less concerned about the health effects," said Dr. Julie Gerberding, the CDC director. 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 was briefed on the situation, and the administration established an interagency team to investigate what Frist told colleagues was a chilling crime.
Gerberding said that although several tests had indicated the substance is ricin, laboratories at the CDC in Atlanta and in Washington were conducting "gold standard" tests that involved inoculating lab animals.
The discovery of the suspicious powder came Monday, in the mailroom of Frist's office in the Dirksen Senate Office Building (search).
Despite the danger, the Senate mounted a show of business as usual, turning to a highway spending bill. But Senate hearings were canceled and all three Senate office buildings were closed.
Capitol police suspended tours in the Capitol itself and advised lawmakers not to open any mail.
Bob Stevenson, a spokesman for Frist, said that in addition to shutting down tours, officials decided to close Senate restaurants and give Senate pages the day off. But he said that essential Capitol employees were expected to report to work as usual.
"This is a very dangerous substance. I think we'll probably be shut down for a couple of days," Sen. Lindsay Graham, R-S.C., told Fox News. "But if the goal of the people who did this was to shut down democracy, they're going to lose."
Meanwhile, forensic scientists were testing a grayish-white powdery substance that turned up at the same Wallingford, Conn., postal facility where anthrax was found in late 2001.
The Connecticut powder was found in a letter addressed to the Republican National Committee in Washington. U.S. postal inspectors told The Associated Press that the powder arrived in a business reply envelope that didn't require a stamp.
A 94-year-old Oxford woman, Ottilie Lundgren, died in 2001 after inhaling bacteria that investigators believe she got through mail that passed through the Wallingford sorting center.
She was one of five people who died nationwide in the anthrax attacks that fall.
What Is Ricin?
Ricin is a highly poisonous substance derived from castor beans. Twice as deadly as cobra venom and easy to make, it can cause death 36 to 72 hours after ingestion, inhalation or injection.
A clue to ricin poisoning is a suddenly developed fever, cough and excess fluid in the lungs, a fact sheet from the CDC says. These symptoms could be followed by severe breathing problems and possibly death. There is no known antidote.
Frist told his colleagues as the Senate opened its session Tuesday that nothing dangerous had turned up besides in his office.
"All air sampling and all environmental studies today are negative, with the exception of what was found in that single office at that site," he said.
Frist also urged calm at a Monday night press conference.
"Nobody is sick, we don't expect anybody to get sick," he said. A surgeon before his election to the Senate, he explained that if symptoms of ricin poisoning had not surfaced in about eight hours, contamination was unlikely.
But, he later added, "The mailroom was in my office .... This is a terrorist activity."
The Homeland Security Department (search) earlier said that it was monitoring the situation, and an FBI official said the bureau was awaiting the result of tests at the Fort Detrick laboratory before deciding whether to get more fully involved in the case.
Gainer said it was not clear what letter or package the substance had come from or how it got into the mailroom.
After it had been discovered, Capitol Police shut down air vents in the building and began evacuating staffers from the fourth floor.
After the first positive test, 16 people who had been on the floor were decontaminated before being sent home. Later Tuesday, the number of those decontaminated went up to between 40 and 50.
"We are also looking for anyone else who was in the area," Gainer said.
Frist encouraged staffers to check the Senate's Web site for updates on the situation. No indication that extra security had been ordered for the Capitol complex was given, although security in the area has been high since the Sept. 11, 2001, attacks.
Mail to congressional offices has been irradiated since the 2001 anthrax attack, but Frist said radiation is unlikely to have an effect on ricin.
Daschle said he remembered all too well what it was like to be in Frist's position. His office and Vermont Sen. Patrick Leahy's office were targeted in the 2001 anthrax attacks. No one was ever arrested.
"I know the difficulty that it presents personally to staff and families of staff," Daschle said on the Senate floor Tuesday. "I know that we're probably in a better position to confront these challenges today than we were two years ago."
Buildings on the House of Representatives side of Capitol Hill remained open, though staffers were handed fliers as they walked in warning them not to open any mail.
"I think we've seen lessons learned from the anthrax scare," former CIA covert officer Mike Baker told Fox News on Tuesday. "The process is in place to react quicker than we did in the past."
Police found traces of ricin in a north London apartment last January and arrested seven men of North African origin.
A package containing ricin was also found at a postal facility serving Greenville-Spartanburg International Airport in South Carolina in October.
Fox News' Jim Angle, Mike Emanuel, Catherine Donaldson-Evans, Jane Roh, Anna Stolley and The Associated Press contributed to this report.