The FBI says the letters sent to President Barack Obama and a U.S. senator are related and are both postmarked out of Memphis, Tennessee, dated April 8.

In an intelligence bulletin obtained by The Associated Press, the FBI says the letters both say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both letters are signed, "I am KC and I approve this message."

The FBI says the substance in both letters have preliminarily tested positive for ricin, a potentially fatal poison.

Both the letters to Sen. Roger Wicker, a Republican, and to Obama were intercepted at off-site mail facilities.

The FBI says it is pursuing investigative leads to determine who sent the letters.