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.
More top news
Missing RAF gunner Corrie McKeague's mother has said he had an account on a swingers' website.
"I will work with him where we can, but I just didn't want to go celebrate or legitimise or normalise his presidency," one US lawmaker said.
Former US President George H.W. Bush is in intensive care after suffering from pneumonia, as his wife is admitted as a precaution.