Paul Kevin Curtis, a Mississippi man, was arrested Wednesday in connection with a letter filled with the poison ricin sent to President Barack Obama and others.
Curtis, 45, was arrested at his home in Corinth, Miss., the US Secret Service said.
The letters to Obama, Republican Senator Roger Wicker and another to a state justice official read "To see a wrong and not expose it, is to become a silent partner to its continuance."
They were apparently signed: "I am KC and I approve this message."
The letter to the president had been intercepted at a screening facility outside the White House grounds on Wednesday.
It was later confirmed that the letter had been poisoned with the deadly substance.
It is not clear whether the letters were connected to the bombing at the Boston marathon Tuesday but the FBI said that no link had been found.
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