Update:
Kevin Spacey, the star of George Hickenlooper's upcoming "Casino Jack" released a statement on Sunday on the director's death at 47. It said, in part:
"I can’t believe he's gone because George was so alive, bubbling with energy, drive, commitment, an open heart and a brilliant sense of humor. He was one of a kind.
"My experience working with him was nothing short of fantastic: from our prison visit with Jack Abramoff, to script meetings, pre-production discussions and finally our first day of shooting. From that day until our last event premiering Casino Jack in Toronto last month, he was a joy to be around. His sensibility and outlook served his everyday. All of us who knew him – who had the chance to collaborate with him – who saw the child in him that he never lost – always looking at life with wonder and curiosity – will miss him with enormous admiration and affection…
"Tonight I raise a glass in his honor. Rest in peace, my friend.
Earlier:
George Hickenlooper, a well-respected filmmaker whose latest film "Casino Jack" is set to open in December, was found dead in Denver on Saturday. 
He was 47. The immediate cause of death was not known.
The distributors of his film, Jonathan Dorfman and Temple Fennell, issued a statement on Saturday afternoon:
"On behalf of ATO Pictures, we are devastated by the loss of our friend and colleague, George Hickenlooper.
Those who knew George can attest to his amazing talent and commitment to excellence. No one held the bar higher or had a bigger heart.
Our thoughts and prayers are with his family."
The filmmaker was not know to be in ill health, and had been energetically promoting his film starring Kevin Spacey as disgraced lobbyist Jack Abramoff – including to this journalist – in the past few months.
He was an avid Facebook user, and his last post there on Friday reflected his excitement over releasing his new film:
"Thanks to all my new friends in Austin for making last night's screening such a great success!" he wrote. "I love Austin! One of the if not the best audience in the world. Literally. Hope to come back soon… On to Denver."
His other films include "Factory Girl" and the documentary "Mayor of the Sunset Strip."
Hickenlooper was in Denver to present 'Casino Jack' at a film festival, and was supposed to present the film Thursday night with his cousin John Hickenlooper, the mayor of the city.
The mayor issued this statement to The Denver post:
HIckenlooper was born in St. Louis Missouri, and began making films at a young age. He received a B.A. in history and film studies from Yale, and made his directing debut with a short documentary about Dennis Hopper.
His real breakthrough came, however, in 1991 with "Hearts of Darkness: A Filmmaker's Apocalypse," a gripping documentary about the making of "Apocalypse Now."
Hickenlooper's friends were devastated by the news. Ben Morgenthau, his roommate at Yale and a lifelong friend, said he was supposed to join the filmmaker to film election night together, as John Hickenlooper is running for governor of Colorado.
Morgenthau was cinematographer on "Hick Town," a documentary by George Hickenlooper about the Denver mayor's office.
"George was such an incredibly loyal friend, he had a huge smile, a huge laugh. He had so much energy to make great films and experience a really big life," said Morgenthau. "He was a magic maker. He’d always say, I’m gonna do this do that, and six, nine months later, it would happen."
A statement from the mayor's office said Hickenlooper appeared to have died of natural causes.
Update:
Here is a selection of comments on Hickenlooper's Facebook page, where hundreds of friends and colleagues responded with shock and sadness:
