A 9-month-old baby in Florida died after his teenage mother allegedly put fentanyl in his bottle before putting him down for a nap, Nassau County Sheriff Bill Leeper said.

Deputies were called Monday to a home in the Callahan area about a baby who was not breathing and had no pulse, Leeper said at a news conference Wednesday. Deputies tried to revive the baby until firefighters took him to a Jacksonville hospital, where he was pronounced dead, Leeper said.

"Originally, the mother said she didn't know what happened. She put the baby to sleep and just couldn't wake him up," Leeper said.

The mother, 17, allegedly "changed her story several times" but ultimately "confessed" she had been tired and wanted to take a nap, filled the baby's bottle, and put some of "what she thought was cocaine" in the bottle, the sheriff said.

The substance was fentanyl, an opioid, according to the sheriff.

The medical examiner's office said the child had 10 times a fatal dose in his bloodstream and determined the cause of death was a fentanyl overdose.

Leeper said the child's death was caused by the mother's culpable negligence and "utter disregard" for his safety.

"The suspect consciously did an act she must have known, or reasonably should have known, was likely to cause of the death, or great bodily injury, of her child," Leeper said.

She was charged with aggravated manslaughter and possession of a controlled substance.

Deputies and the Florida Department of Children and Families are investigating.