A letter addressed to U.S. Republican Senator Roger Wicker of Mississippi tested positive for the deadly poison ricin on Tuesday.

The letter was post marked from Memphis, Tennessee, but had no return address. It was intercepted by authorities before it reached the Senator's office.

Senator Jeff Flake said the incident is cause for alarm.

(SOUNDBITE) (English) SENATOR JEFF FLAKE, REPUBLICAN FROM ARIZONA, SAYING:

"Obviously that's concerning. You know it's not just members concerned. Rarely does it get to the member before it goes through a lot of staff. That's a big concern obviously for all of us. So we're very anxious to get more details."

Officials say all mail to the Senate has been stopped for the time being.

Ricin is a lethal poison found naturally in castor beans.