Several preliminary tests confirmed that the substance discovered in a mail handling room was ricin and results of more extensive tests conducted in a laboratory confirmed the earlier findings, US Capitol Police Chief Terrence Gainer said.
"Two of those three tests do indicate that it is ricin. So we have had several confirmations that it is ricin," Gainer said.
About 16 people who were on the floor where the toxin was discovered were being decontaminated, Gainer said.
Senate Majority Leader Bill Frist, a physician, joined Gainer at the news conference and stressed no one had been injured.
"Nobody's been hurt and everybody is fine. There've been absolutely no injuries whatsoever," said Frist, a Tennessee Republican.
"There is no cause for alarm," Frist said. He said the powder that was found could theoretically have been inhaled, but there was no evidence of that by any of the people who had been in the office or on the floor where the substance was found.
Gainer said the US Capitol Police department was notified by a postal worker in the Dirksen office building shortly after 3pm (2000 GMT) of a suspicious white powder in a mailroom.
He said it was not immediately clear what package or what letter might have held the powder.
"Nobody's been hurt and everybody is fine. There've been absolutely no injuries whatsoever"
Ricin is a poison derived from the pulp left over when castor beans are processed to make castor oil.
There is no antidote for ricin, which can kill within 36 to 72 hours of exposure to significant amounts, according to the US Centre for Disease Control and Prevention.
A vial of ricin was delivered to a postal distribution centre in Greenville, South Carolina on 15 October in an envelope with a letter threatening to widely release the deadly poison unless new rules for commercial truck drivers were changed.
US authorities in January offered a $100,000 reward for information leading to the arrest of whoever sent the toxin.
In 2001, Capitol Hill was one target in a series of anthrax attacks that killed at least five people on the East Coast, including two Washington postal workers.
Letters containing anthrax spores were mailed to the Washington offices of two senators and to news media offices in New York and Florida.