(CNN) -- The body of actor James Gandolfini was transferred to a hospital morgue in Rome early Thursday, where it awaits 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.
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.
Gandolfini was not alive when he arrived by ambulance at the Policlinico hospital late Wednesday, said spokesman Antonio Spasola.
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 suffering from panic attacks.
He was 51.
"This is young to have a heart attack, even when you look at any pre-existing health conditions," said CNN's chief medical correspondent Sanjay Gupta. Usually, first heart attacks don't strike until the victim is over 60.
But risk factors such as obesity, smoking, lack of exercise and stress can cause them to occur earlier in life, Gupta said.
Tributes poured in from colleagues, friends and fans on social media.
Even after Sopranos fame, he was very down to earth, said iReporter Pat Tantalo, who worked on the set of a 2006 movie Gandolfini starred in.
They exchanged telephone numbers and spent some of their evenings sitting around together on a deck by a lake smoking cigars.
"It was like we had been friends forever," Tantalo said.
Gandolfini was well-respected by his peers.
"He was a genius," said David Chase, creator of "The Sopranos." "Anyone who saw him even in the smallest of his performances knows that. He is one of the greatest actors of this or any time."
Gandolfini is survived by his wife, Deborah, and their 9-month-old daughter, Liliana. He is also survived by a son, Michael, from another marriage.
CNN's Hada Messia and Livia Borghese contributed to this report
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