LOS ANGELES - THE probe into Michael Jackson's death was stepped up on Thursday, as investigators subpoenaed medical records and a leading police officer refused to rule out homicide.

Coroners called for any records held by Jackson's many doctors to be presented, 'including radiology and psychiatric records,' the Los Angeles Times reported, part of an ongoing investigation into the singer's death aged 50 on June 25.

Los Angeles police chief Bill Bratton said his department was looking into Jackson's 'prescription drug history, the doctors that he's dealt with over the years.

'Are we dealing with homicide? Are we dealing with an accidental overdose? What are we dealing with? So as we are standing here speaking, I can tell you, I don't have that information,' he told CNN.

Investigators have apparently zeroed in on a powerful sedative - Diprivan - discovered at Jackson's rented mansion after his death.

The drug is commonly used to induce unconsciousness in hospital patients ahead of major surgery and experts say it should only be administered by a trained anesthesiologist.

Jackson's long-time dermatologist Arnold Klein told CNN late on Wednesday that he had been aware the singer had used the drug in the past.

'I knew at one point he was using Diprivan when he was on tour in Germany,' he said. 'He was using it with an anesthesiologist to go to sleep at night and I told him he was absolutely insane.' But he denied earlier being the source of the drugs.

'I didn't give him this crap that they're talking about,' he told ABC television in an earlier interview. 'How am I going to prescribe Diprivan when I don't understand how to use it?'

Mr Klein, who said he treated Jackson three days before his mysterious death, suggested that any doctor found to have supplied the star with drugs should be prosecuted.

Reports have also emerged of police records that include allegations Jackson used Xanax - another powerful sedative - and at one time took as many 30 pills each night. -- AFP