Police investigating Presley Gerber's suspected overdose interviewed other people living at the rehab facility where he died as they try to find the source of the alleged drugs that potentially killed him ... TMZ has learned.

Sources close to the investigation tell TMZ ... the probe is in its early stages, but one of the first orders of business is finding out if Presley took drugs, and where he got them.

We're told authorities are looking into whether or not Presley was prescribed the drugs he allegedly took or if he got them from a dealer or friend ... and finding the potential source is key.

Our sources say cops have already conducted interviews with folks who were at the rehab facility when he died.

We're told police still suspect there was no foul play involved ... and there are no suspects at this time.

We broke the story ... Presley died at a luxury rehab facility on Sunday morning ... and the Santa Monica Police Department tells us they responded to a suspected overdose call at about 9:35 AM and found him unresponsive.

Cops tell us Presley was pronounced dead at the scene about 10 minutes later, after receiving life-saving measures.

Presley, a model like his sister Kaia Gerber, was the only son of Cindy Crawford and Rande Gerber.

The autopsy has been completed ... and his cause of death has been deferred as the investigation plays out.