OKLAHOMA CITY (AP) — A federal drug investigator testified Wednesday that a former Oklahoma doctor facing murder charges in connection with the overdose deaths of several patients prescribed more drugs than any other physician in the state.

U.S. Drug Enforcement Administration agent Mary Surovec said investigators “saw vanloads” of patients pull up at William Valuck’s pain management clinic in south Oklahoma City and that the parking lot outside was always full of vehicles.

“The waiting room was full, jam packed,” Surovec testified during a preliminary hearing for Valuck, who also faces drug distribution charges. “Dr. Valuck was the No. 1 prescriber in the state.”

The former osteopathic physician faces first-degree murder charges in Oklahoma County in connection with the deaths of eight patients from overdoses of drugs he prescribed them. Valuck has pleaded not guilty.

At the start of the hearing, the prosecutor, First Assistant District Attorney Scott Rowland, dismissed a ninth murder count and 15 drug counts that involved another patient who allegedly caused a fatal traffic accident while on drugs Valuck had prescribed.

Surovec said federal and state authorities began investigating Valuck in February 2012 after receiving telephone calls from pharmacies and the family members of some of his patients raising concerns about the amount of prescription narcotics he was prescribing.

Valuck’s patients died from overdoses of the same drugs he prescribed to them, including the narcotic painkillers hydrocodone and oxycodone, as well as alprazolam, Valium and Soma. Authorities have said Valuck would prescribe as many as 600 pills at a time.

“Basically we were getting phone calls from pharmacies and concerned citizens,” Surovec said. “We started hearing about one or two deaths. They were dying from multidrug toxicity”

Some pharmacies even faxed to authorities copies of prescriptions Valuck had written that the pharmacies refused to fill.

“That is very rare,” Surovec testified. “He was giving two painkillers in large quantities.”

Family members of some patients who died from overdoses attended the hearing, which will determine whether crimes were committed and if there is probable cause Valuck committed them.