Singer Michael Jackson has died at 50 after going into cardiac arrest earlier this afternoon, according to several sources.

The LA Times says that he arrived at UCLA Hospital in Los Angeles in a deep coma after suffering a heart attack. A spokesman for the Los Angeles Fire Department said, "The call came in because a person was not breathing. When the team arrived, they saw that CPR was already in progress by someone at the home. The person not breathing was transported to UCLA Medical Center and remains there."

Jackson has been preparing for his series of sold-out 50 concerts (held between July 13, 2009 and March 6, 2010) in London. In a March press conference, he said, "I just want to say that these will be my final show performances in London. This will be it. When I say this is it, this will be it."

Update: Music producer Quincy Jones said, "I am absolutely devastated at this tragic and unexpected news. For Michael to be taken away from us so suddenly at such a young age, I just don't have the words. Divinity brought our souls together on `The Wiz' and allowed us to do what we were able to throughout the 80s. To this day, the music we created together on `Off the Wall,' `Thriller' and `Bad' is played in every corner of the world and the reason for that is because he had it all -- talent, grace, professionalism and dedication. He was the consummate entertainer and his contributions and legacy will be felt upon the world forever. I've lost my little brother today, and part of my soul has gone with him."

Governor Arnold Schwarzenegger expressed his condolences, "The world has lost one of the most influential and iconic figures in the music industry. From his performances with the Jackson 5 to the premiere of the 'moonwalk' and 'Thriller,' Michael was a pop phenomenon who never stopped pushing the envelope of creativity. Though there were serious questions about his personal life, Michael was undoubtedly a great entertainer and his popularity spanned generations and the globe. Maria and I join all Californians in expressing our shock and sadness over his death and our hearts go out to the Jackson family, Michael's children and to his fans worldwide."

Jackson's brother Jermaine spoke at a press conference, "My brother, the legendary king of pop Michael Jackson, passed away on Thursday, June 25, 2009 at 2:26 p.m. It is believed he suffered cardiac arrest in his home. However, the cause of his death is unknown until results of the autopsy are done." Full remarks here; video below: