Two officials told The Associated Press on Wednesday that the letter was being treated in the same manner as a separate one sent to Mississippi Sen. Roger Wicker and was undergoing field tests.
The officials spoke on the condition of anonymity because the investigation is ongoing.
The letter to Wicker, a Republican, was intercepted at a Senate mail facility just outside Washington and has tested positive for ricin. Sen. Claire McCaskill has said authorities have a suspect in mind in that case, though no one has been charged.
It was not immediately clear which senator the second letter was addressed to.
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