A 9-month-old baby boy died last month in Florida after his teenage mother is accused of putting enough fentanyl into his bottle to kill 10 people, officials said Wednesday. 

The 17-year-old mother was arrested on charges of aggravated manslaughter and possession of a controlled substance, Nassau County Sheriff Bill Leeper said during a news conference. She allegedly confessed to investigators that she thought the fentanyl was cocaine, and put it into her baby's bottle to put him to sleep because she was tired and wanted to take a nap, he added.

"Unfortunately, some babies are born to individuals who have no business being parents," Leeper said.   

On June 26, deputies responded to a residence on Deerfield Country Club Road in Callahan, which is about 20 miles north of Jacksonville, Leeper said. They arrived to find the baby boy unconscious on the living room floor. He was not breathing and did not have a pulse. 

A deputy performed CPR and the baby was taken to a hospital where he was pronounced deceased, Leeper explained. 

"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. 

On Monday, investigators received a report from the medical examiner that fentanyl had been found in the baby's blood, the sheriff said, and his cause of death was determined to be a fentanyl overdose. 

Two milligrams of fentanyl can be lethal, depending on a person's body size, tolerance and past usage, according to the Drug Enforcement Administration. 

"The amount he had in his system would kill approximately 10 people," Leeper said. 

The baby's bottle was located and tested, and deputies found signs of fentanyl inside, Leeper disclosed. The bottle has since been sent to the Florida Department of Law Enforcement for further testing.

Investigators brought the mother in Tuesday for an interview, Leeper said, adding that she changed her story several times before ultimately confessing. 

She told officers she had filled the bottle with formula, then went to the bathroom where she found a pill bottle containing what she believed to be cocaine, according to Leeper. She added some to the baby's bottle. 

"She laid him down in his crib to go to sleep and he never woke up," Leeper said. "Who does that? What mother would do that? That's not normal. That is sick. It's beyond my imagination why a mother would do that to her child."

There were others in the home at the time of the incident, but investigators believe it was solely her decisions that led to the baby's death. Officials have not released the names of the mother or her child. 

Leeper added that while the mother was being booked into jail, she indicated she may be pregnant again. 

Leeper also urged struggling parents to reach out for help, saying there are resources available. 

In:
• Fentanyl
• Florida

Aliza Chasan

Aliza Chasan is a digital producer at 60 Minutes and CBS News.

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