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.
ICYMI .