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 after falling ill at a hotel in the Italian capital.
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."
Though the cause of death is not confirmed, his managers also believe that a heart attack killed the man who portrayed Tony Soprano, a washed-up mob boss prone to keeling over from panic attacks, at age 51.
Before Italian authorities can release Gandolfini's 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 the actor's family has contacted the morgue yet, morgue spokesman Antonio Spasola said Thursday morning.
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.
A medical emergency
Michael Kobold, who described himself as a close family friend of Gandolfini, gave more details of his final moments in a news conference Thursday at the Boscolo Exedra Rome Hotel, where the actor was staying.
Gandolfini experienced a medical emergency about 10 p.m. Wednesday, he said.
His family found him in his room.
"The hotel notified emergency crews who responded, and first aid was administered before Mr. Gandolfini was taken to the hospital, where he died of an apparent heart attack," he said.
"Our prayers and condolences go out to Mr. Gandolfini's family and friends."
Kobold said he'd known the actor from New Jersey for more than a decade and lived with him years ago. "I consider him like a big brother," he said, as he appealed for privacy for Gandolfini's grieving family.
The suddenness of Gandolfini's death struck many people at home and around the world.
New Jersey Gov. Chris Christie was among them, saying the news was "an awful shock."
Heart attacks can happen at any age. But usually, first heart attacks don't strike until the victim is older than 60.
"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.
However, 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.
Edie Falco, the actress who played Tony's wife, Carmela, said Gandolfini was a "man of tremendous depth and sensitivity, with a kindness and generosity beyond words."
"I consider myself very lucky to have spent 10 years as his close colleague. My heart goes out to his family," Falco said in a statement Thursday. "The love between Tony and Carmela was one of the greatest I've ever known."
Former CIA chief Leon Panetta, played by Gandolfini in the movie "Zero Dark Thirty," also issued a statement about the actor Thursday.
"James Gandolfini was a friend and a great actor. He wrote me after portraying me last year, which was a great thrill and honor. I told him I was glad an Italian played me -- swear words and all. We laughed together at the fact that tough guys can have a heart of gold. He did, and we will miss him."
Martin Dempsey, chairman of the Joint Chiefs of Staff, also mourned the "Sopranos" actor.
"Yesterday, we lost a talented actor and advocate for our service members. Our thoughts and prayers are with the Gandolfini family," a statement from the general said Thursday.
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."
He lit up the room with a big smile
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 Livia Borghese reported from Rome and Ben Brumfield wrote in Atlanta. CNN's Hada Messia contributed to this report.