The investigation into the death of Michael Jackson is being treated as possible manslaughter, according to a search warrant relating to the pop star's personal doctor filed with the Los Angeles courts.

Speculation that the inquiry is moving in a criminal direction has been confirmed with the court documents that grant detectives the power to seek "property or items constituting evidence of the offence of manslaughter that tend to show that Dr Conrad Murray committed the said criminal offence".

Murray was the Las Vegas-based physician whom Jackson employed to care for him during the build up to and completion of his London series of concerts that had been scheduled for this month. The doctor was at the singer's side when he collapsed on June 25.

A clinic in Texas and a lock-up used by Murray were raided by officers of the US drugs agency and LA police on Wednesday. The court documents show that among items seized were 27 tablets of a weight loss drug called phentermine, one pill of clonazepam, a muscle relaxant and two computer hard drives.

The raids come ahead of results which are due to be announced by the LA county coroner's office of the toxicology tests on Jackson which are expected in the next week or two. The focus appears to be on the anaesthetic propofol that was found in Jackson's rented mansion after his death. Paraphernalia associated with the administration of the sedative, including a stand holding an intravenous drip and an oxygen tank, were also found in the room.

Murray is registered to practise in California, Nevada and Texas. There have been reports that medical authorities are investigating his practice with a view to suspending his licence.

Several other doctors used by Jackson in recent months have also been the subject of police interest, although none of them have as yet been interviewed by officers.