Federal drug investigators have been called in to find the doctors who prescribed drugs to Michael Jackson before his death.

The inquiry by the Drug Enforcement Administration is thought to make arrests and possible charges much more likely. Police are looking at the wider circle of medical staff who helped to feed the singer's prescription drug habit.

It is claimed Jackson used a "slew of aliases" to obtain powerful drugs including Demerol, a morphine-style painkiller. His death last week at the age of 50 is suspected to have been caused by taking a cocktail of such drugs.

It has been suggested that Jackson was spending as much as $30,000 (£18,000) a month on prescription drugs. One Los Angeles pharmacy, located in the same building as the pop singer's dermatologist Arnold Klein, sued Jackson for an unpaid bill of $100,000 (£61,000).

The authorities have removed medications and two bags of "medical evidence" from the rented house in Holmby Hills, near Beverly Hills, where the pop star was found dead. Police told the LA Times that investigators were interviewing doctors.

Conrad Murray, Jackson's personal physician at the time of his death and the man who discovered the collapsed singer, was questioned for three hours.

His lawyers say he is just a witness in the investigation and never injected Jackson with drugs in the hours before he died. The move comes as Jackson's ex-wife Deborah Rowe was reported to be considering launching a custody battle for their two children. Her lawyers could challenge the temporary custody of Prince Michael, 12, and Paris, 11, granted to the pop star's 79-year-old mother, Katherine. MsRowe is not the mother of seven-year-old Prince Michael II.