Sylvester Stallone at Sage's funeral

Sage Stallone died of a heart attack.

The 36-year-old son of Sylvester Stallone and his first wife Sasha Czack was found unresponsive on July 13 at his Studio City apartment and the Los Angeles County Coroner has determined there was evidence of heart disease when he died.

Despite earlier allegations of drug use, there were none found in his system at the time of death.

A source close to the coroner told gossip website TMZ, it was ''unusual'' for a person Sage's age to have such advanced coronary problems.

An initial autopsy on Sage's body proved inconclusive.

Following Sage's tragic passing, Sylvester, spoke out about his loss.

He said: ''When a parent loses a child there is no greater pain. Therefore I am imploring people to respect my talented son's memory and feel compassion for his loving mother Sasha.''

He also ''begged'' for the memory and soul of Sage - who played the actor's onscreen son in 1990 movie 'Rocky V' - to be ''left in peace'' amid speculation of how he died.

He added: ''This loss will be felt for the rest of our lives. Sage was our first child and the center of our universe and I am humbly begging for all to have my son's memory and soul left in peace.''

© Bang Showbiz