Michael Jackson, the onetime child star turned King of Pop, died in Los Angeles on Thursday afternoon after suffering an apparent cardiac arrest in his home. He was 50 years old.

Jackson was pronounced dead at 2:26 p.m. at the Ronald Reagan UCLA Medical Center not far from his rental mansion in the Holmby Hills. The cause of death wasn't known. An autopsy is set for Friday.

"It's a very sad day for all of us," Jackson's manager, Frank DiLeo, tells PEOPLE. " It's such a terrible shame. He was my friend." As for Jackson's three children – Blanket, 7, Paris, 11, and Prince Michael, 12 – DiLeo says, "The kids are very upset, but they are fine."

The entertainer was stricken Thursday afternoon and a 911 call was placed. "His personal physician, who was with him at the time, attempted to resuscitate Jackson, as did paramedics who transferred him," according to a statement from the hospital.

Frantic, Futile Efforts
With an oxygen mask over his face, Jackson was carted to a red ambulance and rushed to the hospital, arriving at 1:14 p.m. "A team of doctors including emergency physicians and cardiologists attempted to resuscitate for a period of more than one hour but were unsuccessful," the statement says.

Jackson had been busy rehearsing for a high-stakes comeback tour, set to start within weeks in England, with dozens of dates scheduled and tickets selling out quickly. To prepare for the rigors of performing, he was training with Lou Ferrigno, the star of the TV show The Incredible Hulk.

Questions About His Tour
There were some doubts among Jackson's circle about his ability to perform his upcoming concert schedule. "Michael is physically just not ready [for the concerts]," a source close to Jackson said last month. "He weighs even less than he did during his trial, which was almost nothing. He's 50 years old."

Famly members, including his mother Katherine, brothers Randy and Jermaine, and sisters Janet and LaToya, arrived separately at the hospital for a tense and tearful vigil. "First, it was chaotic [in the ER] and then absolute silence," says family lawyer Brian Oxman. "Everyone is just heartbroken. I'm heartbroken."

During the ordeal, Jackson's three children were left with a nanny, and plans were made for them to stay with Jackson's mother for the time being. "Mrs. Jackson loves the kids and she will care for them now," says Oxman.

By the evening, the family left the hospital in five-vehicle caravan for his mother's house in Encino. A teary-eyed LaToya was behind the wheel of a black Mercedes; she was followed by a grim-faced Janet in a droopy suede cowboy hat driving a tan Maserati. Jackson's father, Joseph, was en route to Los Angeles from his home in Las Vegas. Jackson's body was flown by helicopter to the Los Angeles County Coroner's Office, where the autopsy will be performed.