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Vermont State Police, Associated Press
This photo provided by Vermont State Police shows Melissa Robitille. Vermont State Police say Robitille and her boyfriend, Walter Richters III put alcohol in her disabled son's IV tube, killing the 13-year-old boy. They were charged Tuesday, Oct. 7, 2014 with murder.

ST. JOHNSBURY, Vt. — A Vermont woman and her boyfriend have pleaded not guilty to charges they put alcohol in her disabled 13-year-old son's IV tube, contributing to his death.

Melissa Robitille and Walter Richters III were ordered held without bail Wednesday on second-degree murder charges in the August death of Isaac Robitille.

Prosecutor Maria Byford says, "There's great evidence of guilt."

Police say Isaac was born with significant medical conditions and disabilities that required the assistance of caretakers, feeding tubes and intravenous tubes.

The couple was charged with using the IV tube to administer alcohol to the boy, leading to his death. An autopsy found his blood-alcohol content was about twice the legal limit for adult drivers.