STROUD, Okla. (AP) — An emergency medical services company in Oklahoma has accepted the resignation its CEO amid an ongoing kickback lawsuit.

Emergency Medical Services Authority, which manages ambulance services in Oklahoma City and Tulsa, has been led by Stephen Williamson since 1978. The company's board accepted his resignation on Tuesday, and he said his retirement would take effect on Thursday.

Chief Operating Officer Jim Winham will take over as interim CEO and president of the company, which serves more than a million people each year, according to its website.

Williamson recently came under fire after being named in a federal lawsuit that alleges a $20 million kickback scheme involving a Texas contractor. He also has been criticized for his compensation and billing practices that caused some users to pay twice for ambulance services.

The company and Williamson have denied wrongdoing.

Williamson's legal expenses associated with the kickbacks lawsuit and another related lawsuit filed by a Tulsa resident will continue to be paid by the company through July. The kickback lawsuit is scheduled for trial on July 23.

Expenses from the lawsuits prompted the company to request an increase its rates in Tulsa. The Tulsa City Council denied the request, saying there's no guarantee that more funding would rescue the agency or the city from eventually having to pay more.

The government is seeking millions of dollars in damages and civil penalties from the company.