The FBI says the letters sent to President Barack Obama and Sen. Roger Wicker are related and are both postmarked out of Memphis, Tenn., 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 Wicker, R-Miss., and to Obama were intercepted at off-site mail facilities.

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

The FBI also says there is no indication of a connection to the bombing at Monday’s Boston Marathon.