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02/08/2008 AT 02:30 PM EST
The Drug Enforcement Administration has subpoenaed Heath Ledger
's autopsy records as part of a federal investigation into whether he was over-prescribed medication, a DEA source tells PEOPLE.
The subpoena, served on the New York City Medical Examiner's Office, is standard DEA procedure when someone dies from an accidental overdose, the source says.
"Usually with an accidental overdose we try to identify if any medication was overly disbursed," the source said.
Ledger, 28, died of an accidental mixture
of prescription drugs, including Oxycodone, Hydrocodone, Diazepam, Temazepam, Alprazolam, and Doxylamin, the Medical Examiner determined.
DEA investigators will interview the physicians who prescribed Ledger the drugs to "make sure there was a medical condition attached to the medication," the source said.
The investigation could take anywhere from five days to two weeks, the source said.