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.
Take ABC13 with you!
Download our free apps for iPhone, iPad, Android and Blackberry devices