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
Newly crowned double Formula One world champion Lewis Hamilton had a phenomenal season. Here is a look at his career and season in numbers.
Work to recover the wreckage of downed Malaysia Airlines flight MH17 has finally finished.
Five political leaders have agreed to tackle young people's questions in a series of live Q&As on YouTube. Here's how you can get involved…