Sylvester Stallone has hired a private investigator to find out how his son died.

The Rocky actor has been left devastated after Sage Stallone - his 36-year-old son from his first marriage to Sasha Czack - was found unresponsive on Friday at his Studio City apartment in Los Angeles, and now Sylvester wants to know what caused his tragic passing.

So the 66-year-old actor, his brother Frank and an official from the Los Angeles County Coroner's Office met with P.I. Scott Ross - who was the lead investigator in the Michael Jackson molestation case and the Chris Brown/Rihanna battery case - at the star's home yesterday, where they discussed Sage's death and the status of the investigation.

According to gossip website TMZ.com, Sylvester wants Ross to figure out what happened to Sage, and even though there is no evidence of foul play, he is not ruling that out. He also wants to know about Sage's final weeks, whether he was using drugs or consuming alcohol, and if there was anything troubling his son.

An initial autopsy on Sage's body, which was completed on Sunday, proved inconclusive and the cause of death is not expected to be known for up to two months until the toxicology tests are completed.