Nick Reiner‘s murderous rampage that resulted in the horrific death of his parents may have been fueled by more than mental illness — meth may have played a big factor.

We sent a camera to Exposition Park near downtown L.A. — where Nick was arrested — to ask people about the drug culture in the area, and whether they ever saw Nick there.

One of the people we spoke with — featured on the most recent episode of the “2 Angry Men” podcast — said flatly … Exposition Park is a drug den where users can get anything they want, including meth. He also said Nick looked familiar.

The murders were shockingly brutal — even to the Medical Examiner’s staff as well as law enforcement. Rob and Michele Reiner were butchered — it was carnage.

Harvey and Mark Geragos go through the markings of a meth murder, with the help of Dr. Drew, who also believes the brutality signals Nick was on a drug like meth.

Geragos says he has recon from the jail where Nick is being housed … he’s almost childlike and doesn’t have a clue why he’s there.

TMZ broke the story … Nick had been stable on a drug cocktail for his schizoaffective disorder until a month before the murders, when he complained to doctors the meds had made him gain weight. The doctors changed his meds and he spiraled out of control.