A federal judge in East Texas has declined to reduce the 18-year prison sentence imposed on a woman for sending ricin-laced letters to former President Barack Obama and others.

The Texarkana Gazette reports that 40-year-old Shannon Richardson appealed her sentence on the grounds of ineffective counsel.

But Judge Caroline Craven issued a ruling Wednesday saying Richardson's claims were without merit.

Richardson, a former actress whose credits include the television series "The Walking Dead," pleaded guilty in 2013 to a federal charge of possessing and producing a biological toxin.

Richardson was arrested earlier that year. Authorities said she tried to implicate her estranged husband after he filed for divorce.

Prosecutors said she mailed three letters outside Texarkana then went to police and claimed that her husband had done it.

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Information from: Texarkana Gazette, http://www.texarkanagazette.com