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
The Royal Marine shot a Taliban fighter in 2011 and had his initial murder conviction quashed earlier this month.
The report found one man had voted twice in the EU referendum and a Tory councillor who had been jailed for submitting a false address.
Weather set to improve with plenty of sunshine by Saturday