When investigators looked closer, they reached a different conclusion: It was the 35-year-old pregnant actress who had sent the letters, and she tried to frame her estranged husband in a bizarre case of marital conflict crossing with bioterrorism.
Those allegations are detailed in court documents filed Friday as Richardson was arrested and charged with mailing a threatening communication to the president. The federal charge carries up to 10 years in prison, U.S. attorney's office spokeswoman Davilyn Walston said.
Richardson, a mother of five who has played bit roles on television and in movies, is accused of mailing the ricin-laced letters to the White House, to Bloomberg and to the mayor's Washington gun-control group last month.
Richardson's court-appointed attorney, Tonda Curry, said there was no intention to harm anyone and noted that it's common knowledge that mail is checked before it reaches the person to whom these letters were addressed.
"From what I can say, based on what evidence I've seen, whoever did this crime never intended for ricin to reach the people to which the letters were addressed," Curry said.
According to an FBI affidavit, Richardson contacted authorities on May 30 and implicated her husband, Nathaniel Richardson. She described finding small, brown beans with white speckles -- a description matching the key ingredient in ricin, castor beans -- at the couple's home in New Boston, Texas. She also told investigators that she had found a sticky note on her husband's desk with addresses for Bloomberg and Obama, the affidavit said.
But she later failed a polygraph test, the document said, and investigators looking into her story found numerous inconsistencies. Among them: Nathaniel Richardson would have been at work when Internet searches tied to the letters were made on the couple's laptop and when the envelopes containing the letters were postmarked.
Finally, the affidavit says, in an interview with authorities on Thursday, Shannon Richardson admitted that she had received syringes and lye -- a caustic chemical used in making ricin -- in the mail; that she had printed the labels for the letters; and that she mailed them. However, she insisted her husband typed them and "made her" print and send them, the affidavit says.
No charges have been filed against her husband. His attorney, John Delk, told The Associated Press on Friday that his client was pleased with his wife's arrest and was working with authorities to prove his innocence.
Delk previously told the AP that the couple is going through a divorce and that the 33-year-old Army veteran may have been "set up" by his wife. In divorce papers filed Thursday, Nathanial Richardson said the marriage had become "insupportable because of discord or conflict of personalities."
FBI agents wearing hazardous material suits were seen going in and out of the Richardsons' house on Wednesday in nearby New Boston, about 150 miles northeast of Dallas near the Arkansas and Oklahoma borders. Authorities conducted a similar search on May 31.
The house is now under quarantine for "environmental or toxic agents," according to a posting at the residence. Multiple samples taken from the couples' home tested positive for ricin, according to the affidavit. Federal agents also found castor beans along with syringes and other items that could be used to extract the lethal poison, the affidavit says.
Bloomberg issued a statement Friday thanking local and federal law enforcement agencies "for their outstanding work in apprehending a suspect," saying they worked collaboratively from the outset "and will continue to do so as the investigation continues."
Shannon Richardson appears in movies and on TV under the name Shannon Guess. Her resume on the Internet movie database IMDb said she has had small television roles in "The Vampire Diaries" and "The Walking Dead." She had a minor role in the movie "The Blind Side" and appeared in an Avis commercial, according to the resume.
Delk said the Richardsons were expecting their first child in October. Shannon Richardson also has five children ranging in age from 4 to 19 from other relationships, four of whom had been living with the couple in the New Boston home, the attorney said.
Nathaniel Richardson works as a mechanic at the Red River Army Depot near Texarkana, Texas, a facility that repairs tanks, Humvees and other mobile military equipment. He and Shannon were married in October 2011.
A detention hearing for Shannon Richardson is scheduled for next Friday, court records show, and the government is requesting that she be held without bond.
The FBI is investigating at least three cases over the past two months in which ricin was mailed to Obama and other public figures. Ricin has been sent to officials sporadically over the years, but experts say that there seems to be a recent uptick and that copycat attacks -- made possible by the relative ease of extracting the poison -- may be the reason.
If inhaled, ricin can cause respiratory failure, among other symptoms. If swallowed, it can shut down the liver and other organs, resulting in death. The amount of ricin that can fit on the head of a pin is said to be enough to kill an adult if properly prepared. No antidote is available, though researchers are trying to develop one.
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