Some US Senate offices have reopened after an alert sparked by the discovery of the deadly toxin ricin in the post room of top Senator Bill Frist.
No poison-related sickness has been reported among staff
Senators and staff were allowed back in to the Russell building at noon, and closure orders for two other blocks are expected to be lifted soon.
No illnesses have been reported and no new traces of the poison found.
Officials are reported to be thinking of extending safety measures brought in after anthrax was sent to senators.
All items sent to the US Congress are screened and have been irradiated since shortly after the anthrax attacks in late 2001.
That kills viruses and bacteria, but does not neutralise a poison like ricin and further safeguards are now being considered, Reuters news agency reported.
The alert was raised after an intern in Mr Frist's office noticed powder on a letter-opening machine on Monday.
Tests indicated it was ricin but no other traces have been found, nor has there been detection of an envelope which might have been used to deliver the poison.
No-one has claimed responsibility for the apparent attack and no connection has been established with other ricin-laced letters sent to the White House and the Greenville-Spartanburg International Airport in South Carolina.
Debates on the Senate floor continued, and senators attended a joint session of Congress to hear an address by Spanish Prime Minister Jose Maria Aznar earlier in the week.
Made from castor beans
May be inhaled, injected or ingested
6,000 times more powerful than cyanide
Initial flu-like symptoms, followed by death within three to five days
No known antidote
Quite easy to make, but more difficult to use as a mass contaminant
Secret agents used it to kill Bulgarian dissident Georgi Markov in London in 1978
However, many hearings and committees were affected and votes cancelled.
Mr Frist - himself a medical doctor - confirmed that none of the workers thought to have been exposed to the powder had displayed any signs of illness.
"I'm happy to report everyone is doing fine," he told reporters on Wednesday.
Health experts said casualties in the attack were increasingly unlikely.
"As each minute ticks by, we are less and less concerned about the health effects," said Dr Julie Gerberding, director of the Centres for Disease Control and Prevention in Atlanta.