Two letters with traces of the poison ricin sent to president Barack Obama and a US senator are related, the FBI has revealed.
Both postmarked from Memphis, Tennessee, dated April 8 and say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both are signed, "I am KC and I approve this message."
The FBI said the substance in the letters has preliminarily tested positive for ricin.
Both the letters to senator Roger Wicker, a Republican, and to Mr Obama were intercepted at off-site mail depots.
The FBI said there was no indication of a connection to the bombing at the Boston marathon.