US Senate hearings were disrupted yesterday after a powder which initially tested positive for the poison ricin was found in a congressional mailroom, stirring memories of a deadly 2001 anthrax attack.
Senate Majority Leader Bill Frist said he was convinced the powder, found on Monday afternoon, was ricin, but an Army laboratory was making tests to confirm this.
In Connecticut, an envelope containing a suspicious powder was found in a post office overnight and officials said it was addressed to the Republican National Committee. The powder was being tested to see if it was ricin. It was unclear if the two cases were linked.
Mr Frist said 16 people were nearby when the powder was found in a Senate mailroom but not all had been exposed and there were no reports of illness.
"We made a decision based on procedures that are quite thorough and comprehensive to close the three Senate (office) buildings today," a Tennessee Republican who is also a physician, said. He was speaking on the floor of the Senate, which, like the House of Representatives, remained open.
The leading Senate Democrat, Tom Daschle of South Dakota, told reporters that so far it appeared the material had been concentrated in one area and had not gone through the ventilation system to other parts of the capitol complex.
"Mail is being collected so that we can determine whether there are other letters that may also contain this deadly poison," he said.
A government official said the incident bore the hallmarks of a domestic criminal action - not a terror attack.
"This does not bear the marks of international terrorism, it appears to be criminal in nature," one US government official said.
Tests were under way on the substance found at the postal facility in Wallingford, Connecticut. It is the same facility where anthrax spores were found in the 2001 anthrax-mailings, which killed five people in different parts of the eastern United States.
Ricin is a potentially fatal toxin with no antidote but experts say it is hard to distribute, making the likelihood that anyone has been poisoned very small.
It is probably most famous for its use in the 1978 assassination in London of a Bulgarian dissident who was injected in the leg with a ricin pellet by a man with a poison-tipped umbrella. The dissident died four days later. The case involved the Bulgarian secret service and the KGB.
Ricin was also discovered in apartments in London in January 2003 that had been occupied by Islamic extremists.
The Ricin scare prompted postponement of a busy day of Senate hearings on the 2005 budget proposal unveiled on Monday by the president. Hearings in the House of Representatives were due to take place as planned. US security services are on alert for possible attacks in America as President George W. Bush pursues a war on terrorism, but there was no immediate indication who was behind the incident.
The three US Senate office buildings, Hart, Dirksen and Russell, remained closed while unopened mail was collected and removed.