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(NBC NEWS) -- Senate mail is on hold while police investigate a ricin scare in Washington, D.C.

The FBI has now confirmed at least two letters, one intended for the president, another for a U.S. Senator, and a mail screening facility have all tested positive for the poisonous substance.

Investigators say they have questioned an individual in Mississippi who they believe is responsible for both letters, but they are waiting to make any arrests until completing a final analysis confirming the suspicious substance is in fact ricin.

Police say the suspicious letters were intercepted in part because of security screening put in place following the anthrax mail attacks back in 2001.

Five people died and 17 others became ill as a result of those attacks.

The FBI says the letters are not connected to the Boston bombings.

All Senate mail is being held until Monday at the earliest while the investigation continues
and offices are being told not to accepted any sealed envelopes.