For many of his fans, Michael Jackson's death came as a shock.
But for those who followed his deteriorating health over the past two decades, the 50-year-old pop icon's demise may have been less of a surprise, rather the culmination of an assortment of health conditions -- including problems with stress and drugs -- that gradually whittled him down to a frail version of his former self.
Acquaintances have said in the past that Jackson was dealing with problems associated with his heavy use of the pain drug Demerol. And while details remain scarce, even before officials announced Jackson's death, the conditions behind his demise suggested a sudden and severe heart emergency.
"His personal physician who was with him at the time attempted to resuscitate my brother... as did the paramedics who transported him to Ronald Reagan UCLA Medical Center," Jackson's brother Jermaine Jackson said during a Thursday press conference. "At approximately 1:14 p.m., a team of doctors including emergency physicians and cardiologists attempted to resuscitate him for a period of more than one hour, and they were unsuccessful."
The true cause of Jackson's death will likely remain a mystery until the coroner's report becomes available. According to a statement form the Los Angeles County Coroner's Office, an autopsy will be performed today.
But those close to Jackson describe a hectic lifestyle.
"I think it was his lifestyle in general," said Stacy Brown, co-author of "Michael Jackson: Behind the Mask" and a family friend. "He had a lot of medical issues. Most of it was brought on by himself."
Kevin McLin, a friend of the family and Jackson's former publicist, agreed that the stress that Jackson put on himself lately led to burnout, which could have contributed to his demise.
"When you continue to try to sustain those kind of standards, you can really bring unwanted stress upon yourself," McLin said. "He had been working non-stop since he was 9 years old."
Drugs May Have Contributed to Poor Health, Friend Says
But Brown described another factor that may have contributed to Jackson's failing health.
"I think we will find that drugs played a major role, prescription and otherwise," Brown said. "From what I understand, there was even some heroin use. There was a lot of morphine. A lot of different drugs."
Jackson himself had admitted to a painkiller addiction in 1993. He claimed that his addiction stemmed from an injury he sustained in 1984, when a special effects explosion during the filming for a Pepsi commercial badly burned his head.
He also claimed that the mental stress of child-molestation allegations fueled his need for medicine.
"I was humiliated, embarrassed, hurt and suffering great pain in my heart," the pop star said in an audiotape statement in 1993. "I became increasingly more dependent to the painkillers to get me through the days of the tour. My friends and doctors advised me to seek professional guidance immediately in order to eliminate what has become an addiction."
Jackson also reportedly suffered from back pain -- a condition that took the spotlight during his much-publicized trial in 2005 for allegedly molesting a young boy. On March 10 of that year, Jackson failed to show up for a court appointment, ostensibly because his back pain became so severe that he had to go to the hospital instead.
Jackson's health appeared to be worsening in recent years. In 2005, McLin said, Jackson was hospitalized due to "dehydration with the flu."
Recently, pictures had shown him being pushed in a wheelchair, appearing frail and gaunt and wearing a face mask while being helped across the street. In December, there were reports that Jackson was on his deathbed, suffering from a rare lung condition, which his publicist denied.
"I don't know what his malady was at that point," McLin said of a photo of Jackson in a wheelchair last year. "When he was so frail like that, he was very susceptible to colds and the flu."
But Jackson's health issues that perhaps garnered the most public were his numerous plastic surgeries, including a widely publicized nose job, and according to his dermatologist he suffered a skin condition known as vitiligo, which gradually robbed him of his pigment.
Messages left with Jackson's plastic surgeon and dermatologist were not immediately returned.
Was Michael Jackson's Heart to Blame?
The chief suspected cause of Jackson's death continues to be cardiac arrest -- a condition in which the electrical signals to the heart become abnormal, making it impossible for it to properly pump blood through the body. In over 90 percent of victims, death occurs.
Jackson had been preparing for 50 sold-out concerts at London's O2 Arena, slated to begin on July 13. It was an endeavor that Brown and McLin said had put the pop legend under an incredible amount of stress.
Dr. Christopher Cannon, a cardiologist at Brigham and Women's Hospital in Boston, told ABCNews.com that stressful situations "definitely" increase the risk for cardiac arrest.
"Increased adrenaline levels and sympathetic nerve signals can increase the risk of arrhythmia and sudden cardiac arrest," he said.
And Brown said that given Jackson's frail physical state, he believes that the additional stress could have pushed the star's body over the edge.
"In preparation for this tour, his body gave out," Brown said. "He put too much on himself, and the pressure I think took its toll. He abused himself so much for so long."
ABC News' Eileen Murphy and Emily Friedman contributed to this report.