New York City’s jails doctor has abruptly resigned amid questions about his medical certification.

Dr. Trevor Parks oversaw the care of 12,000 inmates at Rikers Island and the Manhattan Detention Center. He resigned last Wednesday.

Parks worked for Prison Health Services. The company has a $123 million contract with the city’s Health and Mental Hygiene Department, which oversees the delivery of medical care to jail inmates.

Parks told The New York Times he quit to focus on appealing the American Board of Internal Medicine’s recommendation to suspend his certification. He declined to elaborate.

The board also declined comment. The health department said it was surprised to learn about the issue.

Dr. Luis Cintron will serve as temporary medical director.