(Adds Tony nomination for "Gods of Carnage", co-star reaction)
By Piya Sinha-Roy and Steve Gorman
LOS ANGELES, June 19 (Reuters) - James Gandolfini, the burly actor best known for his Emmy-winning portrayal of a conflicted New Jersey mob boss in the groundbreaking cable TV series "The Sopranos," died on Wednesday vacationing in Italy. He was 51.
Gandolfini, whose role as Tony Soprano made him a household name while transforming the HBO network and ushering in a new era of drama on American television, had been scheduled to attend the closing of the Taormina Film Festival in Sicily on Saturday.
He died of a possible heart attack in Rome, HBO spokeswoman Mara Mikialian told Reuters.
Since "The Sopranos" ended its six-season run in June 2007, Gandolfini appeared in a number of big-screen roles, including "Zero Dark Thirty," a film about the hunt for Osama bin Laden, and the crime drama "Killing Them Softly."
At the time of his death, he had been working on an upcoming HBO series "Criminal Justice." HBO declined to elaborate on the series other than to say that it was in development and that Gandolfini was a part of it.
He had two motion pictures due in theaters next year.
"We're all in shock and feeling immeasurable sadness at the loss of a beloved member of our family," the network said in a statement. "He was a special man, a great talent, but more importantly, a gentle and loving person who treated everyone, no matter their title or position, with equal respect."
Gandolfini began his career as a stage actor in New York and went on to earn a Tony nomination for his role in the original 2009 Broadway cast of the black comedy "God of Carnage." But he gained fame and broke ground with his signature portrait of the title character in "The Sopranos," playing the head of a fictional New Jersey mob family.
Although he shared the character's Italian-American heritage and New Jersey roots, the actor was known for a reserved demeanor off-camera and generally shied away from publicity.
As Tony Soprano, Gandolfini created a gangster different from any previously seen in American television or film. He was capable of killing enemies with his own hands but was prone to panic attacks. He loved his wife, Carmela, played by Edie Falco, and was a doting father, but he carried on a string of extramarital affairs.
He regularly saw a therapist, portrayed by Lorraine Bracco, to work out his anxiety problems and issues with his mother.
By the start of the show's final season, Gandolfini suggested he was ready to move on to more gentle roles once his TV mobster days were over.
"I'm too tired to be a tough guy or any of that stuff anymore," he said. "We pretty much used all that up in this show."
The program, which earned Gandolfini three Emmy Awards as best lead actor in a drama series, was considered by many critics at the time the finest drama to have aired on U.S. television.
The series was a major factor in establishing HBO, a pay-cable network once focused on presentations of feature films, as a powerhouse of original dramatic television and in shifting the kind of sophisticated storytelling once reserved for the big screen to TV.
The show won the Emmy as best drama series in 2004 and again in 2007 after its final season. The series concluded with a final episode that strongly suggested Tony was about to be murdered before abruptly ending mid-scene, cutting from a shot of Gandolfini's face to a blank screen.
His role also paved the way for a parade of popular prime-time shows built around profoundly flawed characters and anti-heroes, from "Dexter" and "Breaking Bad" to "Mad Men" and "Nurse Jackie."
David Chase, creator of "The Sopranos," paid tribute to his former star in a statement remembering him as "a genius" and "one of the greatest actors of this or any time."
"A great deal of that genius resided in those sad eyes. I remember telling him many times, 'You don't get it. You're like Mozart.' There would be silence at the other end of the phone," Chase recounted.
Actress Marcia Gay Harden, his co-star in "God of Carnage," saluted Gandolfini as a "great partner, masterful actor and a loving, generous human being." Susan Sarandon, who played his wife in the 2005 romantic comedy "Romance and Cigarettes," remembered him in a Twitter posting as "One of the sweetest, funniest, most generous actors I've ever worked with."
And Steve Carell, a co-star in the comedy "The Incredible Burt Wonderstone," tweeted, "Unbelievably sad news. A fine man."
New Jersey Governor Chris Christie likewise hailed Gandolfini as "a fine actor, a Rutgers alum and a true Jersey guy."
Gandolfini is due to appear on the big screen next year, playing the love interest of comic actress Julia Louis-Dreyfus in the film "Enough Said" and a role in a New York crime drama called "Animal Rescue." Both are set for U.S. release by News Corp-owned studio Fox Searchlight.
Gandolfini preceded his career as a performer by working as a truck driver, bouncer and nightclub manager in New York City before he went to an acting class with a friend and got hooked.
"I'd also never been around actors before," he told Time magazine, "and I said to myself, 'These people are nuts; this is kind of interesting.'"
Born in Westwood, New Jersey, Gandolfini was raised in a working-class, Italian-American family by a father who was a bricklayer and high school custodian and a mother who worked in a school cafeteria.
In an interview on the television program "Inside the Actors Studio," he said his parents spoke Italian in the home when they did not want the children to understand them.
"So they didn't teach it to my sisters or myself," he said.
Gandolfini had a son, Michael, with his first wife, Marcy Wudarski, whom he divorced in 2002. In 2008, he married model Deborah Lin, who gave birth to a daughter, Liliana, in 2012. (Reporting by Piya Sinha-Roy; Additional reporting by Bill Trott, Alex Dobuzinskis and Lisa Richwine; Writing by Steve Gorman; Editing by Cynthia Johnston and Stacey Joyce)