The FBI is on high alert, and police are out in force in Washington, D.C.

The FBI has confirmed that a letter addressed to President Barack Obama was intercepted earlier today by the Secret Service and tested positive for the toxic chemical ricin.

Ricin is a poison derived from castor beans, and is so deadly that just a few grains of the powder can kill a person.

The FBI and Washington Post report the letter was intercepted at a remote mail processing facility where all mail addressed to the White House is initially screened before moving to a second processing area.

These governmental facilities have monitoring equipment and the letter was flagged as suspicious. It's been sent to a military installation in Maryland for further testing. This news comes on the heels of suspicious packages sent to Alabama's Sen. Richard Shelby (R).   Sen. Roger Wicker of Mississippi also received a letter that tested positive for the toxin Tuesday.

Since Obama took office in 2008, the White House has received a half-dozen such letters. Similar threats with ricin and anthrax-tainted mail were also investigated at the White House in 2004 and 2001.

Authorities did not say if they have any suspects.