A Vermont daycare operator was arrested Monday in the death of a six-month-old baby who police said fatally overdosed after being given an antihistamine to help her sleep.

Stacey Vailancourt, 53, of Rutland, was taken into custody on suspicion of manslaughter and cruelty to a child arising from the January death of six-month-old Harper Rose Briar, Vermont State Police said in a news release.

“Toxicology testing determined that Harper Briar had high concentrations of diphenhydramine in her body,” the news release said.

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The autopsy report noted that diphenhydramine, the active sedating ingredient in over-the-counter antihistamines, is not to be used on infants without a doctor’s order, according to the news release. There was no such order for Harper.

Vailancourt ran a licensed daycare out of her home.

Police said Harper’s parent left her with Vailancourt, who called 911 when the baby became unresponsive.

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The baby’s obituary in the Rutland Herald said she was daughter of Marissa Colburn and Blake Briar, and was the light of their lives and brought joy to everyone who met her.

“Harper Rose quickly gained the title of 'Perfect Princess,' her beautiful smile made everyone instantly fall in love with her,” it said.