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
From children's scooters to chainsaws, National Trading Standards has listed some of the potentially dangerous items it seizes daily.
A study by the University of Oxford forecasts an increase of more than half a million foreign-born people in England between 2011 and 2014.
More deprived areas of England have received the biggest cuts over the last five years, according to an influential think-tank.