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
Ulster rugby stars Paddy Jackson and Stuart Olding are to stand trial on charges of rape, a Belfast court has ruled.
The Syrian city of Raqqa has been liberated from Islamic State militants, a senior commander for a US-backed force has said.
Paranoid schizophrenic Jeffrey Barry stabbed Kamil Ahmad before removing his penis in the attack at his flat in Bristol last year.