A pregnant woman from the US state of Texas has been charged with threatening President Barack Obama after allegedly sending a ricin - tainted letter to him.Shannon Guess Richardson, 35, who is an actress and mother of five appeared in a court in the city of Texarkana on Friday after being arrested earlier in the day. The 35-year-old first accused her husband of mailing the ricin-laced letters as officials say she contacted the Federal Bureau of Investigation (FBI) on May 30 to implicate Nathaniel Richardson. However, according to an FBI affidavit, investigators detected a number of inconsistencies in her story. She also failed a polygraph test. Richardson is said to have admitted sending the letters afterwards, knowing they included ricin, but she claimed that her husband had typed them and made her mail them. No charges have been filed against Richardson’s husband whose attorney says might be set up by his wife since the couple is going through a divorce. The threatening letters were mailed to the White House, New York City Mayor Michael Bloomberg and the mayor’s Washington gun-control group. They were sent from Shreveport, Louisiana, on 20 May, without a return address. On June 5, FBI agents in protective suits searched Richardsons’ house in nearby New Boston, about 241 kilometers (150 miles) northeast of Dallas near the Arkansas and Oklahoma borders. At least three cases have been investigated by the FBI over the past two months in which ricin was posted to the US president and other public figures. If swallowed, ricin can result in death by closing the liver and other organs. It is also said that a tiny amount of ricin that can fit on the head of a pin is sufficient to kill an adult if appropriately prepared.