ALBANY, N.Y. — Of all the mysteries surrounding last year’s deadly limousine crash in upstate New York, the most central is this: Why didn’t the vehicle slow down?

On Thursday, that question gained more focus after reports that the limousine operator had brought the vehicle in for brake service in the months before the crash, but the repair shop had not done the work.

The condition of the limousine — and whether the company had been grossly negligent in overseeing its care — may be key factors in a criminal case against the company’s operator, Nauman Hussain, who is facing manslaughter and criminally negligent homicide charges.

The vehicle careered down a hillside in Schoharie, just west of Albany, killing 17 passengers, the driver and two pedestrians it struck. It was the nation’s deadliest transportation accident since a 2009 plane crash near Buffalo killed 50 people.