EXCLUSIVE

Bodybuilding superstar Rich Piana would snort lines of pre-workout powder like it was cocaine ... this according to the death report obtained by TMZ Sports.

We broke the story ... investigators found white powder at the apartment where Piana suffered a medical emergency back in August. He died 18 days later.

Law enforcement spoke with Piana's girlfriend to try and find out what the powder was -- and she told investigators it was CON-CRET PRE workout powder (Lemon-Lime) ... which is advertised as a high-caffeine booster.

Piana's GF told cops, "He would snort this powder" ... and she does not believe it was anything illicit.

So, what did cause Piana's medical emergency? Officials say the autopsy was essentially inconclusive ... though it revealed Piana had heart disease.

Unclear if the 20 bottles of steroids found in his home contributed in any way either.