The 36-year-old son of Hollywood actor Sylvester Stallone was found dead in his home last Friday. Police said there were no signs of foul play.
The New York Post has reported a prescription drug overdose is suspected, but it is unclear whether it could have been accidental or a suicide.
CNN said an autopsy conducted Sunday proved inconclusive and a cause of death won't be announced for about two months after officials review the results from toxicology tests.
Sage Stallone's attorney, George Braunstein, told KNBC-TV, Los Angeles, the actor, director and producer was engaged to be married and was working on various film projects at the time of his death.
He appeared alongside his father in the 1990s movies "Daylight" and "Rocky V."