A former actress has pleaded guilty to sending ricin-laced letters to President Barack Obama and New York Mayor Michael Bloomberg, under a deal that her lawyer has said would cap prison time at 18 years.

Shannon Guess Richardson entered her plea in federal court in Texarkana, Texas, to a federal charge of possessing and producing a biological toxin.

Richardson was arrested in June after authorities said she tried to implicate her ex-husband, Nathan Richardson, after he had filed for divorce. Prosecutors say Shannon Richardson mailed three letters, then went to police and claimed that her husband had done it.

The letter to Mr Obama, according to a federal indictment, said: "What's in this letter is nothing compared to what ive got in store for you mr president."

Prosecutors say investigators noted inconsistencies in Richardson's statements and later learned that she had bought materials online to produce ricin, a toxin that can cause respiratory failure if inhaled.

Richardson, 35, has had minor roles in the television series The Walking Dead and the movie The Blind Side. She also is the mother of six children - including one child born prematurely while she was in custody this year.

Her lawyer, Tonda Curry, said last month that she and prosecutors agreed to a deal capping Richardson's sentence at 18 years. Prosecutors say Richardson faces life in prison for the charge to which she pleaded guilty.

A federal judge ultimately will sentence Richardson at a later sentencing hearing, which has not yet been scheduled.

"Shannon is anxious to admit her role in ordering the components to make the ricin, her role in the letters that contained the ricin, and to tell the government who else was involved in those offences," Ms Curry said.

She did not say more about Richardson's possible motives or whom she might name.