It's been 9 months since the tragic death of Audrina Patridge's 15-year-old niece, Sadie, and cops are working to find out who may have supplied the drugs that killed the young girl.

Law enforcement sources tell TMZ ... Sadie's death has officially been ruled a narcotic overdose. However, they've not yet determined the manner of death as there's currently an investigation for possible criminal charges.

Our sources say cops want to figure out how Sadie was able to obtain the drugs.

One source tells us ... Sadie was able to buy Percocet through Snapchat, and it was laced with fentanyl. Of course, if that's the case ... cops will work to determine who sold it to the young teen and charges will likely follow.

You'll remember, the man who supplied Mac Miller with a fatal dose of fentanyl was sentenced to 17.5 years in May of last year after Mac's 2018 death ... so it's very much a serious offense.

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