John Stanton/Getty Images
02/29/2008 AT 03:05 PM EST
Federal agents still don't know how actor Heath Ledger
obtained the powerful painkillers Oxycontin and Vicodin, a source tells PEOPLE.
No bottles for those drugs were found in Ledger's apartment, though bottles were located for the other prescriptions such as sleeping pills and anti-anxiety drugs found in his system
, the source said.
Drug Enforcement Administration agents returned Thursday from interviews with two doctors
– one in Houston and one in Los Angeles – who met with Ledger and prescribed drugs
other than the painkillers, according to the source.
"We're still trying to find out who gave him the Oxycontin and Vicodin," the source said, adding that Ledger did not appear to have any medical condition that would have required those two drugs. "We'll search databases to see if we can find out and also credit cards records and so forth. Either someone gave it to him or he bought it himself or he got it online."
The agents will continue to try to find physicians who may have prescribed the drugs, which is always the first step in such investigations, the source said. After all the physicians have been interviewed, agents then move on to interviewing other people who could have given him the drugs, which contributed to his death.
Soon after the cause of his death was determined, the DEA launched an investigation
into whether there were legitimate medical conditions attached to the all the drugs found in his apartment and in his system.
While relatively small amounts of Oxycontin and Vicodin were found, the source said, "Just the mix itself with the other drugs is enough to kill you."
What's puzzling, though, the source says, is that it appears Ledger had kicked his habit and "everyone spoke so highly of him," so it's not clear why he would have been using the powerful painkillers.
The source adds that the investigation could take another month.