(Capitol Hill-AP) Feb. 6, 2004 - As investigators seek the source of the ricin that was found in a Senate mailroom, legislative staffers who may have been exposed are being asked to help advance medical knowledge.

A spokesman for Senator Frist says nine workers have been asked to give blood samples to Navy researchers. The group was told the samples were to determine if they had developed antibodies to ricin. That might help researchers looking for an antidote.

Two of the three US Senate office buildings have been reopened for business in the wake of the poison scare. Senators and thousands of their aides returned to the Russell Senate Office Building shortly after noon on Thursday. The Hart Senate Office Building reopened several hours later.

The Dirksen Senate Office Building remains closed. It's where ricin was found in the mailroom of Senate Majority Leader Bill Frist. The Dirksen building is to open Monday, although Senate Minority Leader Tom Daschle says it could be opened as early as Friday.

Capitol Police are going through each office building and removing unopened mail to ensure there are no more poisoned letters. The Capitol mail system has been shut down until officials can guarantee it is safe.

Since testing has been negative throughout the mailroom, investigators now must consider if the ricin was placed on the machine by someone or if it had spilled out of an older letter and been there for a long time.

Investigators say they have determined no link between the ricin delivered to the US Senate mailroom and an incident last fall at a Greenville postal facility.

The Centers for Disease Control and Prevention provided additional confirmation Wednesday through genetic testing that the powder was, in fact, ricin. No illnesses have been reported.

According to the Centers for Disease Control and Prevention, ricin is made from the waste left over from processing castor beans. The CDC says people can breathe in ricin mist or powder and be poisoned.

Depending on the type of exposure, such as injection, the CDC says as little as 500 micrograms of ricin could be enough to kill an adult. That dose would be about the size of the head of a pin.

Ricin is twice as deadly as cobra venom. It can cause fever, cough and chest tightness within eight hours after being ingested or inhaled. Death can come between 36 and 72 hours after exposure. There is no antidote.

updated 8:55am by Chris Rees