The US Senate mail facility has been closed for the next two to three days following the receipt of a letter that tested positive for the poison ricin, the Senate Sergeant at Arms told Reuters.
The envelope was postmarked from Memphis, Tennessee, had no return address, and was not "outwardly suspicious" the Senate sergeant said.
The FBI, US Capitol Police, and other agencies are involved in the investigation of mail testing positive for ricin, he added.
An envelope sent to Republican Senator Roger Wicker of Mississippi tested positive for ricin, Senator Dick Durbin told reporters after a group of lawmakers were briefed by the FBI.
Ricin is a lethal poison found naturally in castor beans.
A spokeswoman for Mr Wicker referred inquiries to US Capitol Police, according to Reuters.
A spokesman for Capitol Police could not immediately be reached for comment.
A letter sent to the US senate office has tested positive for the poison ricin, reuters has reported.
US senator Durbin said he was briefed by the FBI on an envelope that test positive for ricin.
The senator said he believed the letter was just one of suspicious letters that have been found.