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 … 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 call lasted only about a minute — and, then cops say Aguilera lept to her death from the 17th floor of an apartment building.

Police immediately started investigating … and, after locating her phone, officers say they discovered a typed-out suicide note written several days before Aguilera passed away.

Stephanie Rodriguez — Brianna’s mother — called BS on the suicide ruling in the immediate aftermath … claiming her daughter was excited about her career in law, and insisting she wasn’t suicidal.

However, Detective Robert Marshall of APD said in a press conference, “Brianna had made suicidal comments previously to friends, back in October of this year. This continued through the evening of her death, with some self-harming actions early in the evening and a text message to another friend indicating the thought of suicide.”