The US Federal Bureau of Investigation (FBI) says a Texas woman has admitted to sending ricin-tainted letters to President Barack Obama and New York City Mayor Michael Bloomberg, but only after trying to blame it on her husband.

Shannon Richardson was charged on Friday with mailing a threat to the president. The federal charge carries up to 10 years in prison.

The 35-year-old actress and pregnant mother of five contacted authorities on May 30 to implicate her husband. She has had small roles in The Vampire Diaries and The Walking Dead, according to her CV on the film database IMDB.

An FBI statement says she failed a polygraph test and investigators found inconsistencies in her story.

The statement says she later admitted mailing the letters knowing they contained ricin, but said her husband typed them and made her do it.

No charges have been filed against her husband Nathaniel Richardson, a 33-year-old Army veteran. His attorney says the couple is divorcing and the letters were a setup.

Ongoing investigation

Agents in protective suits searched her home in New Boston on Wednesday.

Bloomberg issued a statement on 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''.

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 increase and that copycat attacks 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.

Source: Agencies