Three Senate buildings will be closed Tuesday following "several confirmations" that a white, powdery substance found Monday in the Dirksen Senate Office Building (search) is the deadly poison ricin, Capitol Police said.

The powder was found by a Senate postal worker shortly after 3 p.m. Monday near the office of Sen. Bill Frist (search), R-Tenn. Initial tests resulted in one positive and one negative for ricin (search). The substance was then transported by the Capitol Police Hazardous Device Unit to a laboratory, where two out of three tests came out positive for ricin.

"There are several confirmations the substance is ricin," U.S. Capitol Police Chief Terry Gainer said in a press conference late Monday night.

Dirksen and the other two main Senate office buildings were to be closed Tuesday for officials to check other mail in the buildings, but the Capitol was to remain open with the Senate convening Tuesday morning as scheduled.

According to a government official, the substance has been taken to Fort Detrick in Maryland for more definitive testing. Gainer, who would not confirm the testing location, said results of the tests would be announced Tuesday morning.

Also speaking at the late-night press conference was majority leader Frist, who urged calm.

"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 have not surfaced in about eight hours, contamination is unlikely.

But, he later added, "The mailroom was in my office .... This is a terrorist activity."

The Homeland Security Department (search) had 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. 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.

"We are also looking for anyone else who was in the area," Garner said.

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

Parts of the Dirksen building were still open Monday night, and Gainer said any decontamination procedures had yet to be determined. "At the moment we're in a wait-and-see position from an analytical point of view in what next steps we may take," he said.

Authorities were also looking for clues into how the substance got into the Senate building in the first place.

"This is a criminal action and will be investigated as such," Frist said.

Frist encouraged staffers to check the Senate's Web site for updates on the lab tests and any announcements regarding closures in the building. 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.

"The Senate will be in business tomorrow," Frist said.

Ricin is a highly poisonous substance derived from castor beans. Easy to make, it can cause death 36 to 72 hours after ingestion, inhalation or injection.

Police found traces of ricin in a north London apartment last January and arrested seven men of North African origin in connection with the virulent toxin that has been linked to Al Qaeda (search) terrorists and Iraq.

A package containing ricin was also found at a postal facility serving Greenville-Spartanburg International Airport in South Carolina in October.

Democrat Tom Daschle of South Dakota was majority leader in 2001 when deadly anthrax was found in letters sent to his and Vermont Sen. Patrick Leahy's offices in the Hart Senate Office Building. No one was ever arrested in those incidents.

Hundreds of Capitol workers, reporters and tourists who were in the Hart building lined up for tests and doses of Cipro and other antibiotics after the anthrax attack. Areas of that building were closed for months for decontamination.

Frist said irradiation would likely have no effect on ricin because the substance is neither a virus or a bacterium.

Fox News' Mike Emanuel, Jane Roh and The Associated Press contributed to this report.