Shannon Guess Richardson, 35, who was arrested on June 7 now faces up to life in prison at sentencing, date of which has not been set yet.
In a statement, the US Attorney John M Bales and Acting Assistant Attorney General for National Security John Carlin, alleged that during April-May 2013, Richardson made several online purchases of items for the purpose of producing ricin.
Ricin is a biological toxin that can be fatal if ingested, inhaled, or injected.
Federal prosecutors alleged Richardson subsequently placed ricin onto threat letters addressed to Obama Bloomberg among others.
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On May 20, 2013, Richardson mailed the three threatening letters from the New Boston, Texas, Post Office.
On May 30, 2013, Richardson travelled to the Shreveport, Louisiana, Police Department and reported that her husband was responsible for the ricin-laced letters.
Richardson was named in an indictment returned by a federal grand jury on June 27, 2013, charging her with threatening the President of the United States, mailing a threatening letter to New York City Mayor Michael Bloomberg, and mailing a threatening letter to Mark Glaze in Washington DC.