Cause of Death
Michael Jackson died from acute propofol intoxication. Propofol is a powerful anesthetic drug normally used in hospitals during surgery. Jackson's personal physician, Dr. Conrad Murray, had been administering propofol to Jackson as a sleep aid at his home. This was an extremely dangerous and improper use of the drug. On June 25, 2009, Jackson stopped breathing, and emergency responders were unable to save him. The Los Angeles County Coroner ruled his death a homicide.
Role of Dr. Conrad Murray
Dr. Conrad Murray was Jackson's personal physician and was present when Jackson died. Murray had been giving Jackson propofol injections to help him sleep while preparing for a concert tour. Using propofol outside of a hospital setting, without proper monitoring equipment, is extremely dangerous and against medical standards. After Jackson's death, Murray was convicted of involuntary manslaughter in 2011 for his role in the incident. He lost his medical license as a result.
Contributing Factors
In addition to propofol, Jackson had other drugs in his system, including lorazepam and midazolam, both of which are sedatives. These additional drugs combined with the propofol to suppress his breathing and heart function. Jackson had struggled with insomnia and pain management for many years, which led to his dependence on various medications.
Investigation and Legal Outcome
The Los Angeles County Coroner's office completed an autopsy and determined that Jackson's death was a homicide caused by the improper administration of propofol. This classification meant that Jackson's death was caused by the actions of another person, though it did not necessarily imply intentional murder. Dr. Murray's trial began in 2011, and he was found guilty of involuntary manslaughter. He was sentenced to four years in prison, though he was released after serving about two years. 
Answer