When the weirder members of Michael Jackson’s family suggested he might have met his end in a sinister fashion even the singer’s most dedicated fans flinched.

Joe Jackson cried foul within days of his son’s death. Jackson’s sister La Toya blamed “a shadowy entourage” of parasitic hangers-on for “murdering” the King of Pop. But it all seemed too convenient when Jackson appeared have driven himself to an early grave through his own addictions and stresses.

Yet three weeks after his death, there are more questions than ever, after it was reported that Los Angeles police have concluded the circumstances of Jackson’s early death might add up to murder after all. The prime suspects are a group of doctors, one or more of whom may have gone a lot further than providing the star with a few extra pills.

The TMZ website, which broke the news of Jackson’s death, said that several law enforcement sources have told them the police have concluded Jackson was killed by an anaesthetic, Propofol, which is so powerful it should only be used in hospitals under very controlled conditions, with heart monitoring.

The sedative is administered intravenously and was given to help Jackson sleep, because he suffered chronic insomnia. Propofol has found popularity as a recreational drug among some medical staff, but its sheer potency discourages most.

Detectives searching Jackson’s home missed the stash of the anaesthetic on a first sweep, but found it with a drip and an oxygen tank after questioning one of his doctors. TMZ’s sources said there was “plenty of powerful evidence” that the drug had been administered by one of Jackson’s doctors.

One of the doctors, Arnold Klein – who is not believed to be suspected of administering Propofol – told CNN this month he had known the singer was using it “with an anaesthesiologist, to go to sleep at night, and I told him he was absolutely insane”.

The police investigation appears to indicate that prosecutors may conclude that a doctor who knew the dangers of administering Propofol should face a second degree murder or manslaughter charge. Detectives are also investigating written prescriptions, after members of Jackson’s staff complained their names were used to obtain drugs.

Los Angeles police declined to comment on reports that it now regards the investigation as a murder inquiry, but an official said detectives were awaiting toxicology reports from the coroner’s office to confirm the cause of death.