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MEMPHIS, Tenn. – The Shelby County Medical Examiner’s Office says a 9-year-old girl found unresponsive in her bed died from a Fentanyl overdose in May.

Memphis police said Kimberly Knox was discovered by her mother and pronounced dead at Le Bonheur Children’s Hospital.

At the time, police said the little girl had a history of asthma and bronchitis, and there were no signs of trauma or foul play.

D’Angelo Knox, who had partial custody of his daughter, said a doctor told him she died from cardiac arrest, but he was never convinced because she never had heart issues.

Kimberly and D’Angelo Knox

“I didn’t feel good about it. I was questioning the whole thing,” Knox said. “My child she’s up playing and running around daily.”

In October, the medical examiner’s officers ruled Kimberly’s cause of death Acetyl Fentanyl and Fentanyl toxicity.

Knox said he has no idea how his daughter got her hands on the illegal drug, and he won’t rest until he finds out who is responsible.

“I don’t understand how she could get so close to something that dangerous,” he said. “This is my way of helping out — getting justice for her.”

Knox said he recently bought a house and was planning for his daughter’s future, and all of that was taken away from them.

“I planned my whole life around her,” he said. “You try so hard to do stuff for the ones you love and never know when they will be gone.”

The medical examiner also said Kimberly’s death was an accident, but Knox said police are still investigating.

The CDC says Fentanyl, an extremely powerful synthetic opioid is up to 100 times stronger than morphine, and other synthetic opioids contribute to most opioid-involved overdose deaths.

To learn more about the CDC’S drug overdose prevention efforts, click here.