Audrina Patridge's 15-Year-Old Niece's Cause of Death of Determined

'The Hills' alum shared on Feb. 14 that her niece died, just a little over a week after her 15th birthday

The cause of death for Audrina Patridge's teenage niece Sadie Raine Loza has been revealed nine months after her death.

The Orange County Sheriff's Department confirmed to PEOPLE that the teen died of a narcotic overdose as first reported by TMZ.

Authorities noted that "there is still an open investigation to determine if there are any possible criminal charges."

Related: Audrina Patridge's Niece Dead Following Her 15th Birthday: 'It Is the Hardest to Say Goodbye'

Patridge, 38, announced her niece's death on Instagram on Feb. 14 alongside sweet photos of the pair together over the years.

"My heart hurts to even write this. My beautiful niece is now in heaven," Partridge wrote in the caption. "I know [it's] not goodbye forever, but it is the hardest to say goodbye for now."

She concluded with a message for Sadie that read: "We will miss you and cherish every single moment we had with you. Rest in peace Say Say!! We love you forever & ever! ✨🤍🙏"

Related: Paying Tribute to the Celebrities Who Have Died in 2023

The Hills alum received support from her former costars in the post's comment sections. "Omgggg this kills me," Kristin Cavallari wrote.

"So so sorry for your loss Audrina. This is heartbreaking," Brody Jenner commented. "Sending love to you and the family."

Patridge's sister — Sadie's mother Casey Loza — preceded Patridge's tribute with a post of her own, which included a selfie of her late daughter.

"Sadie Raine Loza has left this Earth for the heavens," she captioned the post. "Writing this was the hardest thing I ever have had to do, not knowing the right words I'll leave you with this, Sadie liked to remind me that the first law of thermodynamics aka (Law of Conservation of Energy) works in all spheres of life…..'Energy cannot be created or destroyed, it can only be changed from one form to another.' "

Casey shared how her daughter's legacy will live on, writing: "Transformation is a better word than death. Her story will save countless lives, her memory will never be forgotten. Rest In Peace you brilliant starseed. You're with your creator. We will miss you dearly. I love you."

Related: Stepfather of Audrina Patridge's Late Niece Sadie Speaks Out After Her Death at 15: 'My First Love'

Sadie's stepfather, Casey's husband Kyle Loza, mourned her death by posting the same selfie of the teenager the following day.

"Baby girl - I will carry your memory with me until I see you again," he wrote in the caption. "I can't stop thinking about you. I'll never stop loving you and I would do anything to laugh with you one more time. You were my first Love."

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Sadie died a little over a week after her 15th birthday, which her mother commemorated on social media with a video of Sadie blowing out candles on a birthday cake.

"I'm so proud to be your mom and I cannot wait to see the insane ancient civilization architecture you're going to construct someday," Casey captioned the video. "I love you so so much my Sadie Raine. ❤️"

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