The findings were contained in a previously sealed search warrant which has been made public in Texas.
The singer died in June from a cardiac arrest at his home in Los Angeles. Police have interviewed his doctor, but he has not been named as a suspect.
There are reports that the coroner has concluded Jackson's death was homicide.
The reports, carried by the Associated Press news agency quoting unnamed police sources, have not been confirmed.
But the BBC's Rajesh Mirchandani in Los Angeles says homicide includes manslaughter, and investigators have been trying to establish if there is a case for that charge.
Jackson's doctor, Conrad Murray, has denied any wrongdoing.
Details of the Los Angeles County coroners' findings were revealed when a search warrant affidavit was made public in Houston, Texas, where Dr Murray has offices.
Dr Murray's offices were raided last month as part of the police investigation into the singer's death.
The coroner's office has not published its findings regarding the singer's death.
Addiction fears
According to the affidavit, the LA chief coroner "had reviewed the preliminary toxicology results and his preliminary assessment of Jackson's cause of death was due to lethal levels of propofol".
The documents go on to say that Dr Murray told police he had been giving Jackson propofol as part of his treatment for insomnia.
But, he said he had been concerned Jackson was becoming addicted to the drug and had been trying to wean him off, using alternative drugs.
But, on the morning of the singer's death, Dr Murray is reported to have relented and given Jackson a lower dosage of propofol after a number of other drugs had not worked.
He left the star alone to make some telephone calls and when he returned Jackson was not breathing, the LA Times reports.
Dr Murray is known to have performed CPR on his patient while the paramedics were called, but Jackson was declared dead when he arrived at hospital.
Bottles of propofol found in Jackson's house show it had been prescribed by several doctors, not just Conrad Murray, but he remains at the centre of the inquiry, our correspondent adds.
Earlier this month, Dr Murray - who was employed as Michael Jackson's personal physician for a series of concerts in London scheduled for July - posted a video message of YouTube to thank his supporters.
"I told the truth and I have faith the truth will prevail," he said in the short one-minute clip.
