"The Sopranos" star's body was transferred to the morgue at Policlinico Umberto I hospital early Thursday morning for autopsy.
Although he would couldn't be certain until the autopsy had been performed, Professor Claudio Modini, head of the emergency department at the hospital, said it was "probably a natural cause of death, myocardial infarction."
Modini said ambulance and emergency staff tried to resuscitate Gandolfini, but "there was nothing to do" when he arrived at the emergency room. They said they had not noted alcohol, drugs or any other suspicious factors.
The U.S. Embassy in Rome must issue a death certificate in order for Gandolfini's remains to be returned to the U.S., and said it "will offer any proper assistance to the family when they request it."
Those who knew the 51-year-old actor said he had plenty of risk factors that could lead to cardiac issues. He enjoyed cigars and was not a small man.
"He loved his food," said Larry King, who spent time with Gandolfini off and on camera for years. "He didn't push back from the table."
Gandolfini was vacationing in Italy and was headed to Sicily to receive an award at a local film festival before his death.