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
An advert for luxury fashion brand Yves Saint Laurent has been banned for featuring a model with a visible rib cage and stick-thin legs.
Every school rated as inadequate by Ofsted will be turned into academies under new laws, the Education Secretary has announced.
The new Teletubbies have been revealed - complete with touch-screens on their stomachs.