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
Police say they have traced four drivers who are thought to have hit the woman, and added that a fifth vehicle may also have been involved.
Alex Rowley said he was "disappointed" that Scottish Labour Party were unable to fully investigate harassment claims against him.
Widespread frost, fog and icy stretches overnight. Rain for most during tomorrow