The Austin Police Department has closed the book on Brianna Aguilera 's death, ruling it a suicide ... but the family still doesn't believe law enforcement.

Tony Buzbee , the attorney for Aguilera's family, tells TMZ they still think Brianna's death was more nefarious ... adding, "The Austin Police screwed things up royally."

Buzbee says there's no credible evidence to back APD's suicide claim, full stop. Facebook / Stephanie Rodriguez

ICYMI ... Aguilera was found dead outside an apartment building on Saturday, hours after attending Texas A&M's football game against the University of Texas with trauma from an apparent fall from a high floor.

Brianna lost her phone earlier in the day, cops say ... and, she borrowed a friend's to call her boyfriend around 12:43 AM. The cal

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