Paul Kevin Curtis, 45, was arrested on Wednesday after letters were found addressed to Barack Obama and Republican Senator Roger Wicker.
Initial tests on the letters, identified at remote facilities, showed the presence of the lethal toxin.
The FBI has said there is “no indication of a connection” between the letters and Monday’s Boston Marathon attacks.
The letters addressed to President Barack Obama and Republican Mississippi Senator Roger Wicker were both postmarked Memphis, Tennessee, and dated April 8.
According to US media citing intelligence sources, the letters read: “To see a wrong and not expose it, is to become a silent partner to its continuance.”
They were reportedly signed: “I am KC and I approve this message.”
The FBI said Paul Kevin Curtis also allegedly sent a third letter positively identified for ricin to a Mississippi justice official.
The contents of the letter intended for President Barack Obama were being sent to an accredited laboratory for further analysis, the FBI said, with results expected in 24 to 48 hours.
Paul Kevin Curtis’ relatives earlier expressed shock at the news of his arrest, describing him as a natural musical performer who would impersonate Elvis Presley and Buddy Holly.
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