ROME (CNN) -- James Gandolfini probably died of a heart attack, according to the head of a Rome hospital's emergency department where the actor was taken.
The body of the actor was transferred to the morgue at the Policlinico Umberto I hospital in Rome early Thursday, where it awaited an autopsy.
By law, medical examiners in Italy are required to carry out the postmortem 24 hours after the body's arrival in the morgue, a hospital spokesman said.
Professor Claudio Modini, head of the emergency department where the actor was taken, said he could not be certain of the cause of death until after the autopsy -- but it was "probably a natural cause of death, myocardial infarction," or heart attack.
Emergency room doctors had not noted any suspicious factors, such as alcohol, on his medical report, he said.
Ambulance staff tried to resuscitate the actor at the hotel, and further efforts were made when he reached the emergency room, Modini said, but Gandolfini was dead when he arrived at the hospital, and "there was nothing to do."
Before Italian authorities can release his remains for transport back to the United States, the U.S. Embassy in Rome must issue a death certificate.
No one from the embassy or Gandolfini's family has contacted the morgue yet, said morgue spokesman Antonio Spasola.
The U.S. Embassy said it was "deeply upset" by the news of Gandolfini's premature death but had not yet received any official confirmation from local authorities.
The embassy "will offer any proper assistance to the family when they request it," its statement said.
Hours before he died, Gandolfini told Mario Sesti by phone how much he was looking forward to the next leg of his Italian vacation.
He was heading for Taormina in Sicily, a quaint town packed with historical architecture and nestled between lush green hills and the azure waters of the Mediterranean Sea.
There, he was to receive an award at a local film festival held before the backdrop of an ancient Roman arena.
Sesti, who heads the festival, was eagerly awaiting Gandolfini's arrival Friday. He was shaken by the news of his death.
Sesti will replace the award ceremony with a tribute to Gandolfini's lifetime achievement.
"He was the American actor who better than anyone else was able to interpret the Italian-American society, with all of its rich contrasts, ambitions, pain, humor," Sesti said.
Possible health risks
Though the cause of death is not yet known, his managers believe that a heart attack killed the man who portrayed Tony Soprano, a washed-up mob boss prone to keeling over from panic attacks.
He was 51.
"This is young to have a heart attack, even when you look at any pre-existing health conditions," CNN chief medical correspondent Sanjay Gupta said. Usually, first heart attacks don't strike until the victim is older than 60.
But risk factors such as obesity, smoking, lack of exercise and stress can cause them to occur earlier in life, Gupta said.
"He loved his food. He didn't push back from the table," said journalist Larry King, who spent time with the actor on and off camera.
Photos and anecdotes indicate that Gandolfini enjoyed cigars.
Though intense outbursts of adrenaline-infused anger were a mainstay of Tony Soprano's character, he was a far cry from Gandolfini.
Jovial, fun, reserved
Colleagues described him as funny, kind and gentle. He had a reputation for often being reserved and quiet. He didn't seek publicity, avoiding media coverage, journalists have said.
Tributes poured in on social media, describing Gandolfini as jovial and likable.
Actor Mark Ruffalo shared his grief on Twitter: "Oh Jimmy. It's a crying shame. You stormed in and out. Your voice like velvet granite. From a whisper to a shout. A comet of a man."
Even after his "Sopranos" fame, he was very down to earth, said iReporter Pat Tantalo, who worked on the set of a 2006 movie Gandolfini starred in.
He lit up the room with a big smile the first time he walked into the production office, Tantalo recalled.
"We instantly connected. He planted his huge mitts on my shoulders and called me a little bull."
Tantalo posted a goodbye tribute on CNN's iReport website with a photo of Gandolfini's arm around his shoulder. Both men are smiling ear to ear.
They exchanged telephone numbers and spent some of their evenings sitting around together on a deck by a lake smoking cigars and telling stories.
"It was like we had been friends forever," Tantalo said.
IReporter Shana O'Neil met Gandolfini when he was shooting in her office in 1994. She remembers him as "Jersey through and through," a trait he did share with mobster Tony Soprano.
But he was so much friendlier, O'Neil said. "I just always think of him as that guy."
His "Sopranos" fame, she said, changed nothing about the way she remembers him.
-- CNN's Hada Messia contributed to this report.
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