Preliminary findings show mechanical or infrastructure issues did not cause a Boston Green LIne subway car to derail, sending nine people to the hospital.

The Massachusetts Bay Transportation Authority said Sunday the confirmed cause of Saturday’s incident “continues to be under investigation and requires thoroughness, focus and time.”

Regularly scheduled subway service was resumed Sunday on the affected branches of the public transit system’s Green Line after a “robust testing of the area,” including inspections of track and signal infrastructure and an interview with the operator of the train that derailed inside a tunnel near Kenmore Square. The model of train car involved in the derailment, manufactured by Italian firm Breda, has been involved in several derailments in the past.

Local emergency officials said none of Saturday’s injuries were life-threatening. A 10th injured individual declined treatment.

Emergency workers said they were told by witnesses that everything went dark and they were thrown around the car.