Sylvester Stallone Hires Famed P.I. To Investigate Son's Death

We've learned famed P.I. Scott Ross -- who was the led investigator in the Michael Jackson molestation case and the Chris Brown/Rihanna battery case -- met Tuesday at Stallone's home with Sly, his brother Frank, and an official from the L.A. County Coroner's Office. Sources tell us they discussed Sage Stallone's death and the status of the investigation.

We're told Sly wants Ross to figure out what happened to his son -- and he's not ruling out foul play, even though he has no evidence of that. Sly also wants to know about Sage's final weeks ... whether he was using drugs or consuming alcohol. The actor also wants to know what, if anything, was troubling his son.