Suspicious Powder Prompts Closure of Senate Office Buildings

According to Gainer, the first of the tests on the powder came back positive for ricin; the second was negative. Three more tests were all positive. After that, three more tests were conducted at an outside laboratory. Two of those three turned out positive, Gainer said.

As the day went on, officials put in place a wide array of security measures. One of the first, it appeared, was a shutdown of the ventilation system that serves the Dirksen building and the Hart Senate Office Building, to which it is joined.

Officials also posted police officers outside the Dirksen building, keeping visitors away. Part of Constitution Avenue, which runs past the building, was shut down.

Capitol Police officers and members of the D.C. Fire and Emergency Medical Services Department remained on the scene for hours, although their presence had diminished by midnight.

Mihalko, of the postal service, also said that inspectors were investigating reports of an unidentified white powder at a postal distribution center in Wallingford, Conn. An employee found the powder in an envelope Tuesday morning, police and environmental officials told the Associated Press.

"That's enough to trigger our precautionary protocol," Wallingford Lt. Glen King said. "The worker found it and deemed the letter to be suspicious. Obviously the letter was isolated." Nobody was taken to the hospital and the facility remained open Tuesday morning, police said.

The Wallingford facility is the same postal center at which investigators found anthrax spores in 2001. The substance found Tuesday has been taken to the state forensics laboratory for testing, officials said.

The discovery of the powder came after several days of intense concern about terrorist threats to aviation. However, federal officials expressed doubt last night that the powder -- even if it turned out to be ricin -- was part of a terrorist plot.

They asserted that ricin, which is made from the castor bean, appears poorly suited to inflicting mass casualties of the kind sought by international terrorist groups such as al Qaeda.

According to materials provided by the federal Centers for Disease Control and Prevention, ricin poisoning can follow inhalation, ingestion or contact with the skin or eyes.

The CDC said that initial symptoms of poisoning by inhalation of ricin powder can occur within eight hours of exposure. The severity of the effects depends on the degree of exposure.

According to the CDC, the likely symptoms of inhalation of ricin would be difficulty in breathing, fever, cough, nausea and tightness in the chest. Frist said "it would be important" for anyone with such symptoms to notify the Capitol physician's office.

In explaining why specialists doubt that ricin would be the instrument of large-scale terrorism, one federal security official said the material is not easily prepared for dispersal in a fashion that would affect many people.

"This does not have the earmarks of international terrorism," the official said. Assuming that the material involved was ricin, he said, "this is more of a criminal issue and is likely to be handled by the FBI."