James Gandolfini died suddenly in Italy at the age of 51, according to HBO. The cause of death was possibly a heart attack. According to Variety, the actor was on vacation at the time. 

The actor was best-known for his portrayal of Tony Soprano on the eponymous show. He won three Emmys for that work. But his career extended way beyond that part: the actor played the CIA director in "Zero Dark Thirty," and had roles in “The Man Who Wasn’t There,” and “True Romance.” He also starred in "In the Loop," A British and American indie satire film that's basically a comedy of errors ending in war. The actor had three current projects in the works: "Fox Searchlight’s “Animal Rescue,” HBO's "Criminal Justice," and CBS's "Taxi 22." HBO sent out a statement confirming Gandolfini's death

 “We’re all in shock and feeling immeasurable sadness at the loss of a beloved member of our family.  He was 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.  He touched so many of us over the years with his humor, his warmth and his humility.  Our hearts go out to his wife and children during this terrible time.  He will be deeply missed by all of us.”

A 2007 profile of the actor explored his connections to the role that made him a household name: ""I'm playing an Italian lunatic from New Jersey," the actor has famously said of his turn as Tony Soprano, "and that's basically what I am." But Gandolfini, who is known to refuse almost all interview requests (including those for the AP piece, which is otherwise quite good).  The AP noted, "While Tony Soprano is a larger-than-life figure, Gandolfini is exceptionally modest and obsessive. He has described himself as "a 260-pound Woody Allen."

After rising to stardom, Gandolfini did significant work helping Iraq and Afghanistan veterans. Here's the actor, talking about the HBO documentary he made after visiting Iraq (via Greg Mitchell): 

Gandolfini is survived by his wife, Deborah Lin, and two children. 

We're adding more to this developing story as it comes in.