Michael Jackson, an incomparable figure in music, dance and culture whose ever-changing face graced the covers of albums that sold more than half a billion copies, died Thursday, shortly after going into cardiac arrest at his rented Holmby Hills mansion. He was 50. He spent much of his life as one of the most famous people on the planet, and to many, his untimely death felt both unthinkable and, oddly, inevitable.

Paramedics found Jackson in cardiac arrest when they arrived at his home shortly before 12:30 p.m., three minutes and 17 seconds after receiving a 911 call. His personal physician was already in the house performing CPR. Jackson was not breathing, and it appears he never regained consciousness. Paramedics treated Jackson at the house for 42 minutes, and he was declared dead at 2:26 p.m. at UCLA Medical Center, about two miles from his home above Sunset Boulevard.

"I got to kiss him and tell him goodbye," said Frank DiLeo, Jackson's manager and friend of 30 years, who was at the hospital. "I lost a very dear friend, someone who I admired, someone who was the greatest talent I ever met or worked with."

Los Angeles police said detectives would launch a thorough investigation of the death. They cautioned, however, that they did not believe Jackson was the victim of foul play and that the investigation was standard after the death of a person with his level of fame. Authorities said they would examine whether Jackson had been taking medications that contributed to his death; an autopsy is expected to be performed today.

Jackson's death was confirmed outside the hospital by his brother Jermaine, who once performed alongside Michael as a member of the Jackson 5, a family act that began in the steel mill town of Gary, Ind., before making it big in the music industry.

Jackson -- who most famously lived in Santa Barbara County at his Neverland Ranch, named for the island where Peter Pan and the Lost Boys were in no danger of growing up -- had taken up residence in a seven-bedroom estate in Holmby Hills, which he was renting for $100,000 a month.

He had come to Los Angeles to rehearse for 50 sold-out concerts at London's O2 Arena, a run of shows that was scheduled to kick off July 13 and had been dubbed "This Is It." The concerts were to have been the start of an ambitious career revival designed to begin wiping out Jackson's staggering debt -- he owed at least $400 million and would have earned $1 million a night -- and return the singer to cultural relevancy.

Jackson's backers envisioned the London appearances as an audition of sorts for a reboot that would go on to include a world tour, movies, a Graceland-like museum, new music and revues in Macau and Las Vegas.

Those close to Jackson have said he had been working diligently to get in shape for his comeback. A year ago, he was gaunt and used a wheelchair, but recently he'd been exercising with a trainer in addition to daylong rehearsals with dancers half his age.

Kenny Ortega, the force behind "High School Musical" and "Dirty Dancing," was brought on as the concerts' director. Jackson was consumed with the project, Ortega said, personally approving every costume and every bit of choreography. Jackson was also thrilled by the notion of keeping the details of the tour secret.

"He was so in love with this project," Ortega said. "When I looked into his eyes, they looked great. . . . Michael was sincerely happy."

Rehearsals were to wrap up early next week. Ortega was leading one of the final rehearsals Thursday afternoon when he received a phone call confirming Jackson's death, which he then revealed to the tour performers.

"People fell to their knees," he said.

Johnny Caswell, a principal at Centerstaging, the Burbank soundstage where Jackson had been rehearsing, watched many of the run-throughs and said he was "absolutely shocked" by the performer's death. Jackson, he said, was "very frail" but approached the rehearsals -- including one the night before his death that witnesses at Staples Center called triumphant -- with boundless energy. "He was working hard," Caswell said.

In order for promoters to get insurance for the London shows, Jackson underwent a four-hour physical with an independent doctor this spring. Randy Phillips, the chief executive of AEG Live, the promoter, said the medical screening uncovered "no issues whatsoever."

Word of Jackson's death ricocheted around the nation. In Gary, Ind., outside his modest childhood home, fans formed a prayer circle. In New York, fans moonwalked outside the legendary Apollo Theater, where Jackson performed when he was 9. According to the Associated Press, so many people tried to verify Jackson's death that Google's computers interpreted the simultaneous searches for "Michael Jackson" as an automated attack.

As the night wore on Thursday, authorities pleaded with fans and onlookers to avoid UCLA Medical Center, to no avail. A large crowd, including some people wearing costumes similar to Jackson's performance outfits, gathered. Across the street, the Sigma Alpha Epsilon fraternity house blasted Jackson's music. A group of young girls sobbed, blocking an emergency room driveway until police ushered them aside.

The crowd continued to swell as authorities flew Jackson's body from the hospital to the nearby coroner's office by helicopter -- an event that was televised live.

"Michael lived a tortured life," said Tommy Mottola, who oversaw Jackson's career for 16 years as head of Sony Music. "With his successes came all the pressures. . . . Imagine living with that stress on a minute-to-minute, day-by-day basis. And that's going on in life from the age of 5 or 6 to 50. It's almost shocking he made it through this long."