JACKSONVILLE, Fla. -

Jacksonville police expect to make more arrests in an ongoing investigation into suspected prescription drug trafficking at a Riverside doctor's office.

Detectives arrested an intern, Heather Gruber, at Dr. Earl Horowitz's office two weeks ago in connection with trying to get a bogus prescription filled without the doctor's permission. Investigators say that's when the floodgates opened and police arrested three more employees at the podiatry clinic on Park Street -- Dana Miller, Tracie Hazel and Debra Coffman.

Meanwhile, Channel 4 uncovered federal documents that show this is not the first time people connected to the facility have been arrested, albeit in an unrelated scheme.

In a warning to area pharmacists regarding the prescription fraud scheme, JSO faxed a letter Wednesday morning asking them to check for any hydrocodone prescriptions filled using Horowitz's prescription license.

"We haven't found that there was any activity recently on that physician, so as far as we're concerned here, we haven't noticed any of those false prescriptions," said Steve Stafford, a pharmacist at Robert's Pharmacy.

Four employees at the diabetic foot clinic are facing drug trafficking charges. Investigators say they obtained at least 150 fraudulent prescriptions for controlled substances using Horowitz's license to prescribe without him knowing.

"The office staff knew how to manipulate the system," said Channel 4 crime analyst Ken Jefferson. "They went in, they knew how to call a prescription in, they knew how to do exactly what they needed to do to get that prescription drug."

Using the state's prescription drug monitoring system, investigators linked three of the employees to the operation after an intern in the office was arrested for doctor shopping, police said. Detectives said the database has been an incredibly helpful tool.

"The database consists of all the controlled prescriptions that are written in the state of Florida and the patient's name, the patient's birth date, a number of other information so we can cross-check those, and we do," Stafford said.

The previous owner of the Jacksonville Foot Health Center was involved in an unrelated Medicare fraud scheme. Federal documents show the FBI raided the office two years ago. The former owner of the clinic, Joann Mayo, pleaded guilty in an insurance fraud scheme in which she filed $3 million in false Medicare claims from 2008-10.

Staff at Horowitz's' office said Wednesday he was too stressed and upset to talk. His office manager who was arrested worked for him for 15 years.

According to the arrest reports, the women arrested didn't just use their own names, they also called in prescriptions using friends' and family members' names.

Horowitz said Tuesday he was surprised by the arrests. Investigators confirmed Horowitz had no idea his employees were involved in the illegal activity. The letter to pharmacists also confirms Horowitz does not prescribe hydrocodone, and says the strongest medication he gives to patients is Tylenol with codeine, which is not part of the investigation.