A Dallas doctor faces up to 20 years in prison after a federal jury convicted him Friday in a pill-mill scheme for illegally prescribing nearly a million doses of painkillers and anti-anxiety drugs.

Carlos Luis Venegas was convicted of conspiracy to distribute a controlled substance before U.S. District Judge David C. Godbey in Dallas.

Venegas, 62, was accused of dispensing drugs with "no legitimate medical purpose," said Erin Nealy Cox, U.S. attorney for the Northern District of Texas.

According to trial evidence, Venegas acted as the supervising physician for a series of sham medical clinics that were fronts for the distribution of hydrocodone and alprazolam.

Several of his co-defendants, including nurses and clinic managers, previously pleaded guilty to their roles in the scheme, Nealy Cox said.

Witnesses said defendants involved in the plan paid homeless and indigent people to pose as patients seeking pain medication. They coached them on how to describe their symptoms, drove them to the clinics and paid for their appointments, the U.S. attorney said.

Files seized from the clinics showed that most exams were conducted without medical testing and documentation of the patients' illnesses was nearly nonexistent.

Patients were usually prescribed a cocktail of medications, including hydrocodone and Xanax, generally for the highest dosages available, Cox said.

A sentencing date for Venegas has not been set.