Police in Los Angeles are treating Michael Jackson's death as a homicide, according to TMZ, the website that broke the news of the singer's death.
The pop icon's family have spoken of "unanswered questions" concerning the role of doctor Conrad Murray in the final hours of Jackson's life, the website said.
Dr Murray was Jackson's personal doctor and was to join the star for the duration of his London comeback tour.
He is known to have administered the powerful sedative Diprivan to Jackson and was with him when he passed away.
The 51-year-old cardiologist's lawyers insist he has been cleared of any wrongdoing.
TMZ.com said "the evidence points to the anaesthesia Propofol as the primary cause of Jackson's death".
Law enforcement sources told the entertainment website there was already "plenty of powerful evidence" pointing to Dr Murray.
The evidence is said to include items found in Jackson's rented mansion, including Propofol, an intravenous drip stand and an oxygen tank.
The drug is used to induce unconsciousness in hospital patients ahead of surgery and experts say it should only be administered by an anaesthesiologist.
The Los Angeles Police Department has already had "multiple conversations with the county district attorney's office", TMZ said.
The website's report comes a day after the Los Angeles county coroner's office issued a summons for the star's dermatologist Arnold Klein.
Coroners said they were seeking "additional medical records" from the physician's Beverly Hills office.
Investigators have been looking into the possible role of drugs in the death of Jackson, who died on June 25 after suffering an apparent cardiac arrest at his home in Los Angeles.
The exact cause of his death is expected at the end of this week or early next week when toxicology reports are released.
