The Texas actress who admitted sending ricin-laced letters to President Barack Obama and then-New York Mayor Michael Bloomberg was sentenced to 18 years in prison Wednesday, the U.S. attorney's office said.

Shannon Guess Richardson, 36, also was ordered to pay $367,000 in restitution.

"Today's sentencing brings an appropriate and just end to what is surely one of our most unusual, even bizarre cases," John Bales, U.S. attorney for the Eastern District of Texas, said in a statement.

In December, the actress pleaded guilty to possession of a toxin for use as a weapon, prosecutors said.

Though the charge carried a maximum penalty of life in prison, prosecutors said at the time that Richardson entered into a plea dealunder which she would not serve more than 18 years.

Three envelopes containing the natural, highly toxic compound derived from castor beans were mailed from Shreveport, Louisiana, in May 2013, after records showed Richardson made online purchases of castor beans and lye, another component of ricin.

Richardson lived in New Boston, Texas, about 90 miles north of Shreveport.

In addition to Obama and Bloomberg, Richardson also mailed a poison letter to Mark Glaze, then-director of Mayors Against Illegal Guns, which Bloomberg founded, an FBI arrest affidavit said. The group's office is in Washington.

The letters, which contained what the FBI described as "very low concentrations" of ricin, were discovered before causing any injuries or fatalities. They also issued a threat: "You will have to kill me and my family before you get my guns. The right to bear arms is my constitutional God-given right. What's in this letter is nothing compared to what I've got planned for you."

What is ricin?

Richardson -- who had minor roles in television shows, including "The Walking Dead" and "The Vampire Diaries" -- initially told FBI agents that her now-estranged husband had sent the tainted letters.

But authorities said Richardson was found to be "deceptive" in a polygraph exam.

Investigators found that her computer storage devices contained the text of threatening letters to Obama and the couple's computer records showed that her husband couldn't have printed them because he was at work at the time, according to an FBI arrest affidavit.

Eventually, Richardson admitted to investigators that she'd mailed the letters, though she claimed her husband had typed them out and forced her to print and mail them, the affidavit said.

Nathaniel Richardson denied any wrongdoing, telling investigators that his wife was "intentionally misleading" them and that she wanted to end their marriage, court papers said.

He filed for divorce in June 2013, and court papers indicated that Shannon Richardson was pregnant. According to CNN affiliate KYTX-TV, Shannon Richardson gave birth to a boy, Brody, in July 2013, four months before her due date.