WASHINGTON, D.C. - The FBI says the letters sent to President Barack Obama and Sen. Roger Wicker are related and were both postmarked on April 8 and sent from Memphis, Tenn.
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 has preliminarily tested positive for ricin, a potentially fatal poison.
Both the letters to Wicker, a Mississippi 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.
ABC News reported that a second letter sent to the U.S. Senate also tested positive for ricin. No other information was released about who the letter was addressed to or when it was intercepted.
The first letter's discovery shook the U.S. Capitol, where several events were canceled Tuesday in response to Monday's Boston bombing that killed three people, injured more than 170 and ignited fresh fears of terrorism. There was no evidence of a connection between the two events.
FBI spokesman Paul Bresson said initial field tests on the substance in the first Senate letter produced mixed results and it was undergoing further analysis at a laboratory. Only after that testing could a determination be made about whether the substance was ricin, Bresson said.
Capitol Police spokesman Shennell S. Antrobus said police were notified that the mail facility had received "an envelope containing a white granular substance."
"The envelope was immediately quarantined by the facility's personnel and USCP HAZMAT responded to the scene," Antrobus said. "Preliminary tests indicate the substance found was ricin. The material is being forwarded to an accredited laboratory for further analysis."
In 2001, anthrax-laced letters appeared in post offices, newsrooms and the offices of then-Senate Majority Leader Tom Daschle, D-S.D., and Sen. Patrick Leahy, D-Vt. Two Senate office buildings were closed during that investigation. Overall, five people died and 17 others became ill. The FBI attributed the attack to a government scientist who committed suicide in 2008.