"Corey Haim's death is yet another tragedy linked to the growing problem of prescription drug abuse," Brown said in a written statement. "This problem is increasingly linked to criminal organizations, like the illegal and massive prescription drug ring under investigation."
Los Angeles County coroner's officials, however, said they have not yet determined what killed the 38-year-old Haim on Wednesday.
State law enforcement authorities said they were investigating the drug ring and how the name of Haim, who battled addiction for years, appeared on the prescription.
Javier Salaiz, an investigator with the state attorney general's office, said authorities have not yet confirmed that Haim actually filled the prescription or if someone was using his name to acquire the prescription.
However, a person familiar with the investigation who spoke on condition of anonymity because the case is ongoing said Haim may have been doctor shopping.
Assistant Chief Coroner Ed Winter said four prescription drug bottles bearing the actor's name were found in the apartment where he collapsed, but all those drugs had been provided by a doctor who had been treating the actor.
The coroner's office has declined to state what medications were discovered, but said no illegal drugs were found.
Winter said no determination had been made about Haim's cause of death, and toxicology tests would not be available for at least a month.
He said he had not been contacted by the attorney general's office.
"It surprises me that Jerry Brown would come out and give a cause of death," he said.
Brown said later in an interview that he didn't know what killed Haim.
The illegal prescription was for the powerful painkiller OxyContin, he said.
"This is a growing and dangerous problem," Brown said.
Christine Gasparac, a spokeswoman for Brown, said the prescription was found through the state's computer database that tracks prescriptions that are filled.
Investigators believe the ring either sells the pads on the street to addicts or to people who then fill out the forms and obtain the drugs for illegal distribution.
Doctors whose names are on the form usually aren't aware that their identity is being used illegally.
So far, authorities have uncovered up to 5,000 fraudulent prescriptions linked to the fraud ring in Southern California.
Haim's agent, Mark Heaslip, said his client's medications were prescribed by an addiction specialist who was working with the actor. He said he thinks, based on what Haim's mother has told him, the actor may have had an adverse reaction to the medication because he was ill.
"I don't think Corey overdosed, not at all," Heaslip said.
Authorities have said Haim was suffering from flulike symptoms in the days before his death.
Seattle-based Heaslip has served as Haim's agent for 18 months but first met the actor, best known for his roles in the 1980s flicks "The Lost Boys" and "Lucas," in November. He said Haim was poised for a comeback and showed no signs of addiction.
"He's never given me a sign of that," Heaslip said.
Plans are being completed for a public memorial for Haim in Los Angeles. The actor is expected to be buried at a private funeral in his native Canada, Heaslip said.
Brown's office has made prescription drug abuse a priority. It worked with the Los Angeles County district attorney's office to bring drug conspiracy charges against two doctors and the lawyer-boyfriend of Anna Nicole Smith.
Brown also has launched a probe of doctors whose names have come up during the investigation into the death of Michael Jackson.