The Senate’s three office buildings are closed indefinitely following the discovery of a powder late Monday afternoon in the office of Majority Leader Bill Frist (R-Tenn.). Initial testing suggests the substance may be the toxin ricin.

The Capitol as well as House office buildings remained open early Tuesday morning; however, all mail delivery has been suspended. Unopened mail in the Senate office buildings is being collected and removed.

“There is no cause for alarm,” Frist urged at a Monday-night press conference prior to the announcement that the buildings would be closed. Staff working in a fourth-floor corridor facing Constitution Avenue were evacuated from the building Monday night.

“We’ve undertaken the procedures of decontaminating some 16 people who were on the floor,” Capitol Police Chief Terrance Gainer said. Ventilation systems were also shut down to prevent the possible spread of the material.

Capitol Police conducted at least eight tests of a white powder, first reported by a postal worker in Room 454 of the Dirksen Senate Office Building around 3 p.m. Monday. Six of the tests registered positive for ricin — a toxin derived from castor beans — prompting police to send samples to Fort Detrick in Frederick, Md., for additional testing. Results are expected Tuesday afternoon.

“This substance is being subjected to scientific testing but has preliminarily been identified as a form of ricin,” a notice on Frist’s Web site states. Frist described the powder as a “bioterrorist agent” at the press conference.

According to the Centers for Disease Control and Prevention, ricin is a “biologic toxin” that can be fatal if it is ingested or injected, although exposure can also occur through inhalation.

“Ricin is considered to be a much more potent toxin when inhaled or injected compared with other routes of exposure,” a CDC Web site states. Although a 500-microgram dose of ricin, equivalent in size to the head of pin, would be lethal if injected, the CDC notes, a “greater amount” would need to be inhaled or swallowed to be toxic.

Symptoms of ricin poisoning, which the CDC states develop within a few hours of inhaling “significant amounts of ricin,” may include difficulty breathing, fever, cough, nausea and tightness in the chest. The poison can prevent cells from producing necessary proteins, and can lead to kidney failure, respiratory failure, circulatory collapse and fluid loss.

“Nobody is sick. We don’t expect anybody to get sick,” Frist said Monday. According to the CDC there is no reliable test available to confirm ricin exposure. The Attending Physician’s Office referred inquires about the ricin incident to the Capitol Police.

An e-mail issued by the House Emergency Communications Center at 3 a.m. Tuesday morning states that no ricin contamination has been identified on the House side, but the Capitol Police have directed staff not to move, open or otherwise handle mail.

“Capitol Police are developing a protocol for the retrieval of mail previously delivered,” the e-mail states. Staff are advised to contact the police at (202) 225-5151 if they have suspicions about any mail items.

The FBI is assisting Capitol Police with the investigation, an FBI spokeswoman said Tuesday morning. Mailing a “threatening letter” is a federal offense, the spokeswoman explained.

Safety procedures for opening mail, including the use of irradiation to neutralize any potentially harmful substances, were implemented on Capitol Hill following the October 2001 incident in which anthrax-laced letters were sent to several offices, including that of Senate Minority Leader Tom Daschle (D-S.D.).

The 2001 incident shut down the Hart Senate Office Building for more than three months, until January 2002. The decontamination process, which required the use of chorine dioxide gas, cost about $27 million.

The irradiation process would likely not affect ricin, which is classified as biotoxin, and according to the CDC is not affected by “extreme conditions such as very hot or very cold temperatures.” Irradiation can kill bacteria, such as anthrax, and other disease-causing germs.

In October 2003, an envelope containing a threatening note and a sealed vile containing ricin were processed at a Greenville, S.C., mail distribution facility. No apparent cases of ricin-related illness resulted from the incident. In January, the FBI, in conjunction with the United States Postal Inspection Service and the United States Department of Transportation, offered a reward of up to $100,000 for information leading to the arrest and conviction of the letter’s author.

The Senate is scheduled to convene at 9:45 a.m. Tuesday and the House will be in session as scheduled. The Senate restaurants are closed and Capitol tours are suspended. According to the Senate Web site, an announcement about the status of the Senate office buildings is expected at 5 p.m. Tuesday.