James Everett Dutschke entered the plea Friday during a hearing in U.S. District Court in Oxford, Miss.
The 42-year-old previously pleaded not guilty and denied sending the letters. He also denied a later charge that, while incarcerated, he tried to recruit someone else to send a ricin-tainted letter.
Dutschke has been jailed since April on charges of sending the letters to Obama, Republican U.S. Sen. Roger Wicker, and a Mississippi judge. The judge was the only one to receive a letter, though she was not harmed. The letters to Obama and Wicker were intercepted.
Prosecutors recommended a 25-year sentence. That sentence would run concurrently with any sentence Dutschke would face in state court, where he's facing unrelated fondling charges
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