There has been an arrest made for the letters sent to Barack Obama and to at least one Senator, both intercepted at an off-site facility and both tested positive for the deadly poison Ricin.
Via a statement on the FBI website:
Today at approximately 5:15 p.m. CDT, FBI special agents arrested Paul Kevin Curtis, the individual believed to be responsible for the mailings of the three letters sent through the U.S. Postal Service which contained a granular substance that preliminarily tested positive for ricin. The letters were addressed to a U.S. senator, the White House, and a Mississippi justice official.
The individual was arrested at his residence in Corinth, Mississippi following an investigation conducted by FBI Joint Terrorism Task Forces in Memphis, Tennessee and Jackson, Mississippi; the U.S. Capitol Police; the U.S. Postal Inspection Service; and the U.S. Secret Service, aided by the following state and local agencies: the Lee County (Mississippi) Sheriff’s Office; the Prentiss County (Mississippi) Sheriff’s Office; the Corinth (Mississippi) Police Department; the Booneville (Mississippi) Police Department; the Tupelo (Mississippi) Police Department; the Mississippi National Guard 47th Civil Support Team; and the Mississippi Office of Homeland Security.
- View earlier press release: FBI Responds to Reports of Suspicious Letters Received at Mail Facilities
In an intelligence bulletin obtained by AP earlier, the FBI said that both letters 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." (Source)