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
Although it may be a dry and bright start for most, thicker cloud and rain will spread in through the day.
US Democrats must heed the warning of the UK's EU referendum vote to avoid a Donald Trump presidency, Kezia Dugdale has said.
Hundreds of people have been evacuated as a fast-moving wildfire has torched its way across 11,000 acres of land near Los Angeles.