Investigators have now found that an "unauthorized" prescription in his name was linked to an "illegal and massive prescription-drug ring," according to the California Attorney General's Office.
"We know there was a prescription pad stolen and there was a prescription written for him using that pad, so it was fraudulent and we know it was filled," AG spokeswoman Christine Gasparac tells PEOPLE.
The L.A. Coroner has confirmed that four prescription pill bottles in Haim's name were recovered from the scene. But it's unknown whether any of those medications were obtained through the alleged drug ring, or whether Haim even knew about the scam.
Although police mentioned an overdose as a possible cause of death, the official determination is pending toxicology tests. Haim's mother says she was told by the coroner her son suffered from pulmonary congestion.
What is known is that Haim's name arose in the course of a major investigation into what Attorney General Edmund G. Brown Jr. called an "illegal and massive prescription-drug ring."
The ring orders prescription pads from vendors using stolen doctor identities, the AG's office said in a statement Friday.
The pads are then either sold on the street to addicts or to people who are paid to fill the prescription and then sell the drugs on the underground market, the statement reads. The doctor whose name is printed on the form is usually unaware that his or her identity has been stolen, the AG adds.
Haim died at age 38 after collapsing in his mother's Los Angeles apartment Wednesday.